Copying this from a reply to a person in the comments claiming this is a scam. Take it for what you will: This may not be worth my time to respond, but I thought about it and other people reading will be severely misled by what you said. So here it is, for them, not for you: The title says AWD, not 4WD. So half of your argument is out the window. The specifics on tire circumference requirements all depend on the design of the system, and generally speaking most 4WD are part time 4WD with no center differential, so we will go with that design. Running around on dry pavement, you don't have to worry because you will be in 2WD, the rear driveshaft only spins, no issue with binding the gears in the transfer case. If you do enter 4WD, big issue with binding in the transfer case except you would only be in 4WD when traction is an issue. Because the tires have an easier time spinning, either the front or rear will slip a bit to equalize driveshaft speed. No issue, again because you are not on great traction dry roads. If you did however try to drive in 4WD on dry roads, you would have a major issue, try it out, but you have been warned. 4WD with center differential, totally different. This would be a "full time 4WD". I can tell you are European or something, and I am not familiar with all those models, so ill list a couple that we typically see in US... Hummer and Land Cruiser, not common vehicles. These have a center differential in the transfer case so they can run 4WD on dry pavement and not bind up on turns. It is very specific not to engage the center diff lock in good traction scenarios. That is because engaging it will make it function like a part time 4WD, there will be no allowance for front to rear driveshaft speed differential. Meaning again the tires have to be able to slip easily to compensate. If not, you are severely binding the transfer case and will cause failure. And Finally AWD, which is what this video is really about. Front to rear drive, commonly driveshafts but often may have a transaxle directly driving front ring gear with a PTO transfer case coming off to rear driveshaft etc. are drastically different. There are many designs, many of which use a clutch pack/viscous coupling between front and rear. So when there is a speed difference between front and rear, the clutches will slip to compensate. That is totally fine, since it has to do that around turns anyway. Clutches wear on these over time, the heat produced is pretty minimal. No big deal. The fluid will cool down on the next straight away anyway. The problem comes when you are going straight on the highway for miles and miles. If your tires are mismatched front to rear beyond the specification, there is a speed differential. This means there is clutch slipping on turns, but also on straights, all the time. It never cools. So it builds up heat over time and this ultimately leads to fluid breakdown and clutch burning.. ultimately transfer case/transaxle failure. So you and other people are taking information for one system and applying it to other systems that are not applicable. As for your request "you want to hear it from an engineer"... Well.. engineer's don't give speeches, they don't generally create helpful videos... they do however author service manuals. So if you want to speak with the engineer, read the service manual. I work with engineers on a fairly regular basis, it is very typical for them to give the information they feel is needed for service, but not all the information. Especially with Japanese manufacturers. So this information from me, came from the service manual, which they wrote, but it is interpreted a bit. Every AWD vehicle will have specifications for tire replacement somewhere, buried in the owners manual, service manual, or similar. It is often very difficult to extract the information you want. I found the 3% specification, and I shared/explained in a way hopefully people can understand. Clearly not you. But I would also like to agree with you on a point. There are millions of people replacing one tire at a time on AWD and not burning a transfer case or clutch pack. I agree with that. Exactly why is hard to say. Maybe the tires are within 3%. Maybe the engineer who specified 3% rounded down from 3.9%. Maybe they wanted to build in a buffer. Maybe its more of a "just in case". Kind of like oil change intervals. Change at 5k miles, and yet I saw brand new cars sold come back with 15k miles oil never changed and never skipped a beat. Some of it may not be an exact science and they picked a better safe than sorry approach. You all are welcome to make your own decisions, but if you are a little more educated and I helped translate for you, then great! Part time (selectable) 4WD, probably not an issue, just be mindful when you drive in 4 wheel drive there should be snow, ice, or mud where the tires meet the road to reduce stress on 4WD components. Full time 4WD (rare), same as part time except you should be good as long as you are not locking the center diff. If you lock the center diff, make sure there is snow, ice or mud traction. AWD hopefully you can find a spec in the manual, if not I gave you an example. Professional recommendation: do a set. I make nothing off it. If you are frugal like me.... do a single tire but same model tire. Or.... do whatever you want, there is a questionable amount of error allowed. All depends on what type of gambler you are.

@nicgili

11 ай бұрын

First time ive ever heard anyone actually mentioned a mismatch spec, thank you. I tend to lean towards slightly mismatched tires being ok but I know my diffs are all open and parts are cheap for old cars :)

@FISHFANATIC95

7 ай бұрын

Thank you for taking the time to explain this in an understandable way!

@frankconley7630

13 күн бұрын

Bro. Thank you very much. You are helping a lot of people. I haven't written that much in as long as i can remember. My used replacement tire is 3/4 inch different which is way under 3 percent.

That’s the best explanation I’ve heard of the relationship between all wheel drive and tires. Thank you.

Your camera work is on point. Great job explaining this importance. Thanks!

@kittumiku

3 ай бұрын

Oscar level camera work :)

I worked at a tire shop mini moons ago we didn't have all-wheel drive cars then so when the guy at the tire shop told me I had to get four tires I thought he was high who ever thought of that but the video was very explainable and I totally understand it thanks so much cuz I thought I was tripping

Underrated vid. Thank you!

Although the premise is true, Tire Rack can now shave down the one tire you need to match your existing tires. Now, you would only do that if you had very good tread on the other three tires, but it's important to note that it is now doable.

Excellent video 👍

Thank you for this clear explanation!

Thanks for the video, I've been meaning to find some way to visually show someone that Tesla cars with AWD (to get that fast acceleration) comes at the cost of wearing down all 4 tires. Just because the drivetrain is differently doesn't mean differentials aren't used, and so being able to visually show how tread level and the differentials interact, per your video, helped convince some colleagues that the fad of achieving that spicey 0-60 times is a nice feeling until they have to think about the maintenance of such an ability.

@terrancecloverfield6791

6 ай бұрын

Yup. Unfortunately Tesla (like Apple) appeals to people that don't think, and worse, refuse to think. Electric power trains definitely simplify maintenance schedules, no doubt there. Differentials are still used though and so all the conventional wisdom of tire rotations still apply.

Very informative video, perfectly explained. I have a vw golf awd, the brake pedal makes a hissing sound when I step on it. I'd like to know what's causing this?

The different size between the front and the back is important because the center diferencial has a silicon viscos clutch no mechanical and it gets over heated with more than 3% different tire size.

@paulkelley86

Жыл бұрын

Yeah this is the exact reason on a typical AWD car. The clutches slip excessively and build up the heat.

Great explanation, thank you.

Thanks Paul, I'm 98% on your side. I will never buy one or three tyres at a time. People seem to forget that if you do kill a tyre you can always put the spare on the road. I never had "good" spare tyres, they are the cast-offs of damaging tyres whilst driving on dirt roads regularly. For me, just as important as killing your diff/s, grip needs to be symmetrical. If you have one new tyre and one at say 50% on the back and you're in the wet, does the average guy know what the car will do on the next corner? My tyres will always be the same make and model on all four corners and more often than not, I buy four. I also learned on my first of six Subaru Outback's, to get a four-wheel alignment done, not just the front. For about $20 extra, the tyre life and handling was greatly improved. Twenty one years and six Outbacks later, if Subaru had kept making manual transmissions, I'd still be in one today.

In other words 4 new tires, of any price, is cheaper than replacing/repairing drive train? The tire wear you showed indicated some pretty nasty differences. I guess sometimes we have to learn the hard way. Monitoring tire pressure and regulat tire rotations at 5,000 mile oil changes is even cheaper than tires. A small air compressor and a couple of hydraulic jacks from HARBOR FREIGHT are cheap too. I can guess what the oil in this car looks like. Thanks for your efforts and the information you provided. I am considering an AWD vehicle and this info was exactly what I was looking for. Happy trails

Thanks. I never heard of the 3% rule. Can you include a video about tire shaving? How, when and who should do it?

@paulkelley86

Жыл бұрын

No manufacturer recommends that. So, no video. Sorry. And 3% was specified on this model, it's not really a rule, it's one example of a manufacturer specification. Which will vary one manufacturer to the next based on design.

@dn6127

Жыл бұрын

@@paulkelley86 thank you!

What about 2 summer tires on front axle and 2 winter tires on rear? My one tire got puntured on the sidewall and i cant find the same replacement

I think the "replace all tires advice" is all a scam. The general rule of thumb is that you don't want more than a 3% differential in size in your tires, otherwise the clutches start to engage and can wear out. So, what is a 3% differential? On a 275/40r20, you're talking a tire whose diameter is more than 3/4th of an inch smaller (28.7 inches vs 27.84 inches). Is wear of that amount on a tire even possible? Well, the tread depth of your standard new all season tire is 10/32nds of an inch. A fully worn tire, i.e., you're down to your wear bars, is at 2/32nds of an inch. So, maximum normal wear is the difference -- 8/32nds, or 1/4th, of an inch. Now, keep in mind that this measurement is relevant to the radius. To compare it to the diameter, multiply by 2. So, the maximum reduction in a tire's diameter from wear is 2 x 1/4th inch = 1/2 inch. Using our 275/40r20 tire example a fully worn tire is down to 28.2 inches (28.7 - .5), which is comfortably within the 3% (27.84") standard. What about a tire with a smaller diameter? A Prius uses a 195/60r15 tire, which when new has a diameter of 24.2 inches. Fully worn, the tire would be down to 23.7 inches. A 3% size reduction is 23.47, so again a fully worn tire even on a Prius is within the 3% permissible spec. Keep in mind the examples above are extremes, I cannot envision a situation where someone would have brand new tires and fully worn tires on their car at the same time. In all likelihood, the max difference would be around 5/32nds of an inch, which means you're even further within the 3% spec. Bottom line, the "replace all tires" instruction is, IMO, a sales scam. I've been driving awd cars since 1987, and drove several cars over 200k miles. Never had a differential fail, and I've never followed the "never replace only one tire" rule.

@paulkelley86

Жыл бұрын

Good input, my 3% spec is from overall experience in lots of service manuals. It is hard to find an exact spec, if it is even provided in the first place. Correction for you though, it is 3% rolling circumference, not tire diameter/height. I think we can agree that a lot of people will take the "better safe than sorry" approach which probably explains what we see... 4 tires, 4 tires 4 tires. But my role is to help people understand the facts and the concepts. So hopefully I have achieved that. Your math applied helps. Keep in mind I have nothing to sell, I'm not in business, certainly not tire business. Nice contribution, I am going to take your words as trying to help people understand, not as an attack on me.

@danielscott9726

4 ай бұрын

Thank you

@user-og5lt7ui7l

4 ай бұрын

All tire shops won’t do only 1 or 2 tires change if you bring all wheel drive car to them. So it’s either you buy used tire in some random used tires shop or you go with what they say.

@MrOsfaso

4 ай бұрын

Thanks for this explanation because I don't believe in this all 4 tires need replacing unless it's really bad

@0rvelazqu

4 ай бұрын

​@@user-og5lt7ui7l you can order a single new tire online and take your wheel with your new tire to any shop and they will install the tire on to your wheel for like 30 bucks with balancing included

I have a rav4 2014 4WD. Do i need to change all four also?

Thanks for the video! I'm still confused as to why damage is caused if all differentials are open type. Is it just that the spider gears will turn too much to compensate for the difference in rotation and lose their bearings from the heat? I understand that the diffs are engineered to take up the difference in rotational speed between the wheels, but have to be missing something. The potential for damage just doesn't make sense to me, but it has to be true since everyone knows about it. I could see it in a locked diff setup. Any clarification would be super appreciated!

@paulkelley86

2 жыл бұрын

It is because there is no differential front to rear commonly. It would be a clutch pack that would cause slip and wear. No spider gears unless it is full time 4wd in which would actually have a center diff.

@RedneckSavant

2 жыл бұрын

@@paulkelley86 Gotcha! Thanks for the clarification. So in a toyota Alltrac system where there is an open center diff (not a viscous coupling as some have), the worry is less than a subaru that contains a clutch pack. That makes sense.

@paulkelley86

2 жыл бұрын

@@RedneckSavant yeah if it's open diff, I wouldn't even sweat the slight difference in tire circumference between newer and worn tires. I'm not sure I would run different sizes, but that comes down to how much is it going to cause those spider gears to walk, a major size difference might overspeed them I'm a way they are not designed. Seems like you understood that already. I can say that viscous coupling center diffs will overheat with pretty minimal tire circumference difference And a part time 4wd diff like a 4x4 pick up will bind up horribly. You got it

Basically don’t buy an all wheel drive vehicle.

This is an excellent explanation. I had a question, your title said AWD. Would this principle apply on a car like a Ford ranger that has 2H, 4H, 4L and 4A?

@paulkelley86

5 ай бұрын

That's what we call a part time 4wd. You are OK to have a little mismatch from front to rear on those since highway driving would be done in 2wd, which means the front is not connected to the rear. If you were in 4wd however (any, 4h,4l etc) you should in theory have identical tires front and rear, what's you have to consider though is you wouldn't or at least shouldn't be in 4wd dry pavement. It should only be 4wd off road or in snow, in which case each tire can slip a bit because the low coefficient of friction on the slippery surface. So it shouldn't put much strain on the 4wd transfer case anyway, it's easier to just force the tires to slip a bit to equalize front and rear driveshaft speeds.

@chandamusonda6883

5 ай бұрын

@@paulkelley86 Okay awesome, thank you for your response 🙏🏾.

@Paul Kelley so does this apply old trucks cuz i got a old 1979 f250 with weld front an rear diffs an i run any thing i can get my hands on. so will this type thing hurt a older car/truck? an i run fwd a lot cuz of the roads i live on

@paulkelley86

9 ай бұрын

Yeah it would be horrible for you to run in 4 wheel drive with mismatched tires on dry pavement or good traction situations. There needs to be a mechanism to make up for speed difference and in a part time 4wd there is not. But your issue isn't really tire related, even running 4 identical tires, you will cause major strain on your transfer case in 4wd with good traction around turns. My question would be: why are you in 4wd? Do you need it? If you don't need it for traction, leave it in 2wd. If you do, that would indicate you must be in mud or snow in which case you I'll have no issues since your tires are pretty easily able to spin/slip on the road surface to make up any speed difference from front to rear on turns.

Hey, could you suggest a bigger tire for my 2020 equinox without changing the rims , this is my current tire size 235/50R19 Keep up the awesome work , thanks

@paulkelley86

2 жыл бұрын

I don't know offhand what will fit. I can help explain how the tire sizes work. 235 is how wide the tire is in millimeters. 50 means the sidewall of the tire is 50% of the width (235) so it is 117.5 millimeters. So, if you are looking for ground clearance only, you could simply increase the 50 to a 55, or 60. Then the tire is small and skinny. Or if you are looking for bigger overall, you could increase the 235 to say 245 or 255. Which will be wider but also taller since it is now 50% of 245 making it 122.5 millimeters sidewall. Or if you are just looking for better street traction and don't want it to be higher off the ground you could go wider to the 255 and drop to a 45% which would keep it similar height at 114.75 mm sidewall. But again none of these are guaranteed to fit, it depends on clearance, especially during turning at the same time as hitting a bump. I would check forums for equinox to see that is already tried and true.

@williamg8853

Жыл бұрын

Paul's response and recommendation are good, but it is more complex. There can be many clearance issues you have to look out at address. A bigger tire can rub the fender well at full lock, or it may interfere with suspension or steering parts. Some customizer solve the problem with wheel spacer to push the wheel out farther. I would also say talk to others who have fitted larger tires...let someone else go through the trial and error and leverage their efforts.

So wat happen if you get a flat(damage beyound repair) on long trip. Can you use a spare to get to the neareat tyre shop? Can you another size wheel for a short term. If so how long?

@paulkelley86

2 жыл бұрын

The key is that the spare needs to have the same (within typically 3%) rolling circumference. As example a full size spare on a truck or suv. No problem assuming the same size. **note occasionally one brand tire vs the next can vary in circumference but usually not beyond a few percent, same with a new tire vs worn tire. The key is the percentage limit as specified by the manufacturer. Some temporary spares, like a donut could be OK but many are smaller circumference and would be a no go. Incorrect circumference will cause a speed difference between front and rear driveshaft, and these vehicles often to do have a mechanism to compensate or allow for substantial front to rear speed difference in transfer case/AWD unit. Typically the side to side difference is less crucial but drastic circumference difference side to side can cause differential failure as well.

I have a AWD 06 Honda Pilot. I only need three new tires. Do you feel I can get different new tires and keep the one odd ball? It’s practically new and don’t want to waste a good tire.

@paulkelley86

Жыл бұрын

Can you buy 3 of the same type as the good one? Not just size, but the exact same tire. That is a for sure go.

I have a 2015 cherokee with all wheel drive. I put larger size tires on it than factory size. from 245 65 17 to 265 65 17. is this going to burn up my differentials? also, I am assuming that because of your explanation it means it is recommended to rotate in a full size spare to equalize circumference in case of a tire failing

@paulkelley86

Жыл бұрын

As long as they all are the same size and close to same circumference, no problems. And you are correct on the spare in theory, although a matching size worn vs. New has never actually caused damage as far as I have heard. Rotating it in is kinda like cheap insurance better safe than sorry type of thing.

@northernstar1869

Жыл бұрын

@@paulkelley86 thanks Paul. I do think this is knowledge that most AWD owners don't possess unfortunately

Would you mind clarifying how tiny or large the 3% difference is? I have different newer set on back and it’s difference brand as well, but same size. What’s the easiest way to know if I’m good or need to replace all 4? Thanks

@paulkelley86

Жыл бұрын

3% is 3%. The way to calculate is to measure rolling circumference. The best way to do that is to make a mark on the tire where it is dead center on the ground, then roll the vehicle forward until the mark is dead center on the ground again. Measure mark to mark and that will be your rolling circumference. You would be close measuring the actual circumference which is literally the circumference of the tire, just like a circle. You could do it by measuring the outer tread in a circle, or using pi radius squared. But neither of these factor how the tire flexes under vehicle load. So they are not preferred. Then put all 4 in a formula to determine percentages of difference between each. The same as any measurement. Ita a lot of work and this as you can see, is why most techs are going to just say "replace all 4". Probably a mix between they don't actually know how to do this, and it is impractical to do. Also it is often hard to find the allowable percentage difference provided by the manufacturer, engineers sometimes have an attitude of "they don't need to know that" and don't provide information like this. But this is the theory.

@almajd.k

Жыл бұрын

@@paulkelley86 Glad there's someone like you who has extensive knowledge and ACTUALLY replies to everyone happily. Regarding the difference in measurement, it seems like a hassle indeed and I can't be bothered so I'll just get a whole new set. Thanks alot again!

@paulkelley86

Жыл бұрын

@@almajd.k thank you, it's my job since my focus is teaching class rather than fixing cars every day. You can't go wrong with the whole set. :-)

@bethanytmeier

Жыл бұрын

Wow this was like a no Brainer to understand when you explain it like that. Thanks!

So can I replace all 4 with a smaller size tire. Example : I have a crosstrek tire size 225 55r17 I'm looking at a set of AT 215 55 17. Would that be OK?

@paulkelley86

7 ай бұрын

No isssue.

So I have a 2023 Mazda CX5 AWD w/ 11k miles. I have 1 tire that might need replacing with small pcs of metal stuck in it. With it being 11k miles on tire would it be safe to do just 1? Thx

@paulkelley86

11 ай бұрын

Ultimately the correct answer would depend on the manufacturer. However in practical application if it is the same tire (brand model and size) I would have no hesitation in doing the one.

How about a Mazda cx5 that’s front wheel drive most of the time and the awd kicks it when the front tires slip ???

@paulkelley86

3 ай бұрын

It's going to be similar. Per the manufacturer it will call for all tires to match. Its still going to use a viscous coupling or clutch pack, it won't have a center differential. But Like people say, many many people get away with a new tire with worn remaining tires. It's just if something does go wrong, the manufacturer has a reason to decline a warranty claim, and a technician would link an awd failure to mismatched tires. So if you are by the book, do 4. If you understand what I explain in the video, make your own decision based on the factors mentioned.

So if your tires are halfway down and you get a flat on one of them, you have to replace all four?

@JustinSchulz0806

Ай бұрын

Yup.

@BrandamnPSN

Ай бұрын

@@JustinSchulz0806 damn. But not a 4wd?

@JustinSchulz0806

Ай бұрын

@BrandamnPSN nope. I just learned this today cause I'm dealing with it on my car currently. Pin in the sidewall of one tire and they want mee to replace all 4 and my car is only 8 months old. 7k miles

What would I have to replace if this does happen My shifting is sometimes ruff and noticeable, transmission?

@paulkelley86

Жыл бұрын

This question is equivalent of asking a doctor what illness you have with symptoms of a runny nose. Tests have to be done. Your issue needs to be diagnosed by someone with the necessary knowledge.

With all wheel drive are you supposed to rotate your tires

@paulkelley86

2 жыл бұрын

The owners manual will specify but generally yes assuming they are the same size all around. I don't know of any besides AWD EV's that can't be rotated due to staggered fitment sizes. When in doubt, owners manual 😁

Question my drive to work has really long right turns. I have calculated that everyday my right side travels 150-300ft more then the left. Please note I go a different way back home to pick up kids from school. So I do a loop. I only replace 2 tires at a time and have been driving AWD suv for 15 years and never had a drivetrain issue. My fronts and backs stay paired with each other. Thoughts?

@paulkelley86

Жыл бұрын

If it ain't broke don't fix it. If you have 150k miles of experience that is tried and true and more valuable than manufacturers guidelines.

@GabeHelma

Жыл бұрын

What vehicle and what style of AWD?

So for the 3% size , are you talking about the aspect ratio/ middle number on a tire size?

@paulkelley86

Жыл бұрын

Rolling circumference

I worked in a tire shop many moons ago so when I was looking for a tire he said I need all 4 tires 1

How long does the damage take to effect the car? How many miles? How much is the repair for such damage? I heard damage is to drive train. Can you clear this up for me? I’ve heard you need to drive it with mismatched tires for months before damage is had and I’ve also heard only a few hours. Please clear this up for me I’m in quite a jam and can’t afford four new tires at the moment and I have bad credit so I need to know if I can get away with driving it a little while as I save for new tires

@paulkelley86

Жыл бұрын

The extent of damage and timeline would depend on the specific vehicle, and it would mostly be proportional to the difference in size. Generally if you are replacing 2 tires with the same size and model tire, you are good. No damage, nothing. Personally that's what I would recommend if you can't afford 4. If you are doing less than 4, I don't generally recommend changing tire models, and for sure not brands. Two different brand tires listed as the same size can vary quite a bit in actual circumference. Even still, I have seen many AWD cars with mismatched tires and no problems. I however recommend the better safe than sorry due to possibility of damage if they are too different. Then what will really bite you is having different sizes front and rear. That's a no go for AWD. It will result in a couple possibilities, but the damage depends on what type of AWD system. Both will be extensive. As for a part time 4WD truck (can shift from 2WD to 4WD, a person could technically run different size tires front and rear as long as they never enter 4WD, and if they do, same advise as above. To clarify damage, almost always the damage would occur fairly quickly. Maybe on the first drive, or hundreds of miles, or even a few thousand depending on the system and the extent of the mismatch. In a gear to gear AWD, that is going to bind and overheat immediately. In a viscous clutch it will progressively get worse with overheating caused by the mismatch. With a clutch pack, it may take a while but it will wear clutches prematurely. All depends on the specifics.

@tomantonsen4505

Ай бұрын

Hi mate wondering if you can help I have a 2012 x3 xdrive 20d judering in low revs on certain gears wondering if it’s save able or if I’m going to have to think about a second hand transfer box ? Any advice would be appreciated

What would happen if I fit two new tires on one axle with two part worn tires on the other axle and and just let some air out of the two new tires and pump the worn tires up a little until the wheels on both axles have the same effective rolling circumference? Hypothetically, I could make a small paint mark on the edge of each tire and drive the car slowly for 100 wheel revolutions while two helpers watch the tires on both sides and count the revolutions. Then I could adjust the air pressures accordingly, but I imagine that could be a time consuming process. Realistically, I would imagine car manufacturers could easily set up OBDII sensors to do the same thing if they wanted to. (Keep an eye on the differences in wheel revolutions I mean, when the car is being driven in a straight line).

@paulkelley86

2 ай бұрын

You get the lesson. That would probably work fine. Also it shows you that a low tire on an AWD could cause the same issue as mismatched sizes. Very good. And regarding the sensors, they do use wheel speed sensors to monitor tire pressure in the "indirect" tpms design.

@hermanfelderhof5305

2 ай бұрын

@@paulkelley86 As old Jed Clampet would say "Well Doggies!" I had to look up "TPMS" on the interet to find out what you were talking about. Now on replaying your video, I see that I missed something important there at 0:38. You said you had a TPMS light on it, but it that went right over my head because I didn't know what TPMS meant the first time I watched. So now I have watched your ASE A4 Class Unit 1 Tire and Wheel with TPMS Systems and I have learned even more. Now it makes sense. I live Australia and cars are expensive here but fortunately I live in a desert climate so they last a long time. I have a 1994 Ford Falcon Sedan which still works great and recently I have also bought a 2011 Subaru Forester (Diesel). All this OBDII stuff and its acronyms and jargon is new to me but I am determined to get on top of it. It seems like I have found just the right KZread channel, you have a new subscriber. Thank you very much for your all your hard work to present all this great content for us all. I'll check my ODBII reader and my Torque Pro app and see if I can access any of that TPMS data from my Subaru.

@paulkelley86

2 ай бұрын

@hermanfelderhof5305 yeah I only know what goes on in the US so I apologize for the acronyms and jargon. As I sometimes say, in America we speak American. Thanks for tuning in! Hope it helps :-)

What is the rule of thumb for the tread depth difference?

@paulkelley86

11 ай бұрын

Same tire size and brand/model I have never seen an issue. I would have no issue replacing one. But Not sure I am brave enough to risk installing a different brand tire unless I measured rolling circumference. Many shops will recommend 4, which is often what the manufacturer recommends. By the book is 4 on an AWD. I would NOT ever install a different size on one.

@fromisheon4867

11 ай бұрын

@paulkelley86 I am talking about in terms of the tread depth gauge. Would you say starting from 3/32 of an inch difference is good call for replacement?

@paulkelley86

11 ай бұрын

@@fromisheon4867 I have not seen any manufacturer spec tread depth difference.

@fromisheon4867

11 ай бұрын

@paulkelley86 If your car's a left front tire is 9/32 and a right front tire is 6/32, what would you do? Replace both tires? Or the right front tire only? What about 7/32 and 4/32? 6/32 and 3/32?

We have a Buick LaCrosse that dealer spent 2000.00 trying to stop it from pulling and eating tires. Still does it. Any clues where to look?

@danbrown935

Жыл бұрын

222000 kms and AWD

@paulkelley86

Жыл бұрын

@@danbrown935 my initial thought is I would like to see the alignment numbers. Just because a place "does an alignment" doesn't really show what the measurements are. Lots of techs set the toe only, or just get it in the green. The specific wear on the tires would also indicate the cause. Alignment wear is different than aggressive driving which is different than rough roads. Hard to say without really getting an overall look at the vehicle and situation. Doesn't sound likely to be an AWD issue.

@danbrown935

Жыл бұрын

@@paulkelley86 was thinking of buying it. I figured that to. Someone just doesn’t know how to align the car. Dealer or no dealer. 2000.00 Canadian I might take the chance on it

@danbrown935

Жыл бұрын

@@paulkelley86 thanks

@bigglyguy8429

Жыл бұрын

I'd look at the vehicle's insurance history, as it may be bent from an accident?

4 reasons why not to buy an AWD. 1) LT Front goes flat. 2) Replace LT Rear because LT Front went Flat. 3) Replace RT Front because LT Front went flat. 4) Replace RT Rear because LT Front went flat. If AWD is going to create this kind of crap? I don't want it! It is only good for take offs and more about driver performance. FWD may not take off as well, but with good All Weather tires and the take off maybe slower in the winter weather, but it drives better in the snow and controls better because the Front tires guide the car in the direction you want to go. In about 3 years I want to upgrade to a RAV4 and it will not ever be AWD!

Hi Paul, I own a Crosstrek like this, a 2016, which was slammed hard on the passenger wheel by a drunk driver, hard enough to buckle the driver-side wing. My view is such a car cannot be repaired well enough, but I'm concerned the insurance company will insist on repairing it. I don't know enough about car repair to be sure - is it even possible to ever get the thing straight enough again? I should add I'm in SE Asia and there are no computerized repair robots or anything, it would be a bunch of young guys with big hammers and bigger smiles, telling me it's OK... I'm not so sure? And how to tell if it's OK?

@paulkelley86

Жыл бұрын

If you were in the area I would say for sure it could be straightened. Collision shop would pull body and frame straight using lasers or tram measurements. Then an alignment tech could do final adjustments during alignment. If it all makes it within spec it is a go. In your area... i can't say for sure. Can't measure with naked eye. They may be using tram measuring equipment though, which could be OK.

@bigglyguy8429

Жыл бұрын

@@paulkelley86 OK, thanks. So there's little I can do visually to check, but I can check to see what sort of equipment they have? For me, for sure, there's no way I'm keeping the car; my wife wants a new Crosstrek anyway. I just don't want to sell someone a lemon.

Therefore, a part time 4WD or part time AWD system will be fine if you replace only two tires on the same axle, assuming you keep it in 2WD mode. My GMC Terrain manual recommends replacing all 4 tires at the same time, but qualifies with "However, if it is necessary to replace only 1 axle set of worn tires, place the new tires on the rear axle." Theoretically, using AWD in snow shouldn't be any more of an issue with tires whose circumference differ from front to back because that would be comparable to the standard ware that a full-time AWD system would experience.

@paulkelley86

Жыл бұрын

I agree with the first paragraph. The second is true but understand this is because the better tires on the rear lower risk of spin out hydroplane in the rain. All cars whether front or rear wheel drive. The 3rd paragraph you are close but not accurate. Same size tires front and brand new worn would have a small percentage difference in circumference. Maybe a few percent. Different size all together? Likely exceeding the awd systems ability to compensate for speed difference between front and rear driveshafts/outputs. Notice the manual doesn't say you can use a different size, they say it is OK if necessary to replace 2 tires (they assume same size always)

@Luuuugn

Жыл бұрын

I have 1 bad tire. Called Audi and they said it was totally Fine to just buy 2 new tires for the front… i dont know about that! Audi allroad quattro 2015. The backtires have like 5mm left

I don’t know…I can’t help but to feel this is a scam made up by the tire companies to sell more tires. I mean look at how different all those tires were from just normal wear and tear so the tires are never really all the same. Now not saying you should go out one new tire on when all tires look like crap and need to be replaced anyway… but if your tires still have a good amount of thread left and you get a flat, I’m not convinced that only changing that one tire is going to cause problems

This isn’t true. Anyone who falls for this is crazy. Most all wheel drive cars are not all wheel drive all the the time. They run as as front wheel only until the the front wheel speed increases during a slipping, like on ice, then the computer transfers power to the rear. Owners manuals will actually say tires need to be replaced in pairs. If you have a problem with a rear you need to replace both rears and visa versa. Do not fall for the tire shop SCAM of you need to replace all four. Read your owners manual and push back at any tire shop that say otherwise. They are only trying to sell you tires. It is interesting that he used a Subaru they are one of the VERY FEW vehicles that actually have all wheel drive all the time.

@dwtubeyou

Жыл бұрын

I think you're accurately describing my Kia Seltos AWD, though I'll double check. But in general, would shaving new tires be a decent idea? It seems strange that I'd have 10/32" tread in the rear (new tire) and 6/32" tread in the front (old tires). Especially if I were only to replace ONE tire so that its tread depth matched the other half of the rear pair ?

@adrianruiz-gonzalez6785

Жыл бұрын

You’re right, that’s is true I’m most cases. However in this case he specifically said “Subaru Manual Transmission”. The AWD manual transmission in the Crosstrek and Impresa use a viscus split center differential which is susceptible to the damage he described. Other than that you’re totally right, and it is a complete scam! In other systems like those in my Toyota RAV4 there is an “on demand” system that only kicks when then the computer activates a rear clutch plate that sends power to the rear wheels.

Don’t ever take a turn more than the 3% it will grenade the whole drivetrain

@paulkelley86

9 ай бұрын

Cute, but every type of AWD system is designed for turns. Having different sized tires is like turning for your entire drive, every time you drive, and never straightening out. They are not designed for that. It is the heat generated by the speed difference that can overheat the fluid, so a few seconds of turning does not create much heat, even the tightest turn you can make. Try going in tight circles for hours a time though, you will see some damage to the clutches in an LSD or in an AWD transfer case. You did catch on to the fact that turning requires differential action due to the front/rear driveshaft speed difference so props there.

In AWD it doesn't matter different sizes it has 3 differentials , in 4WD it matters as it splits 50 percent power to the front axle and 50 percent to the rear axle and have only 2 differentials so it can't correct any different in tires speed

@paulkelley86

Жыл бұрын

No. SOME AWD have a center diff. But that is not the majority. Like I explained, it depends on the specific vehicle design, which is why the manufacturer provides a specification. 3% allowable difference front to rear is the most common I see. There are 4WD that have center differential as well. Not common but the Hummer in my garage has one, that is not common, mainly on rare full time 4WD. Your statement may very well cause people do damage viscous coupling in their AWD because you are familiar with a specific type, applying it incorrectly to all. Respectfully.

@X-zk9vm

Жыл бұрын

@@paulkelley86 ur right if anyone isn't sure about their vehicle design its better to stay on the safe side , for me I check for the 3rd differential by turning the steering wheel all the way to any direction on a dry pavement if the vehicle moves with ease as if its 2wd then it has a 3rd differential if not it isn't, also love the hummers

Very informative and very disappointing. Thank you.

Yeah pass. I’m not changing 4 wheels from a nail that can’t be repaired.

Sorry, I want an engineer who builds these to explain this to me. How on earth can a couple of mm of treasure depth be enough to wreck a transfer box? There's gonna be millions, maybe tens of millions of 4wd vehicles on the road. Forget tread depth - tyre pressure alone can account for a difference of rolling circumference. Lose a few PSI in one or 2 wheels and you're going to have a noticeable difference. In that case people would be writing off transfer boxes in such high numbers the cars would be deemed not fit for purpose. AND what about actual off roading, where people may have 1 or even 2 wheels off the ground entirely? AND what about the fact that whenever turning the inner snd outer wheels are moving at different speeds? For the record, I own a 4wd. I wear an axel out every year and I replace those tyres every year. 7 years. 62k miles. Cars on 120k now. Never had a problem with the 4wd or transmission. There are millions of people doing the same. Sometimes car parts go wrong. People look for a reasons why. I bet people who have religiously wasted money on new tyres to do all 4 have suffered transfer box failures. I also bet people like me who change 2 haven't. I have even changed 1 on occasion as they're also run flats. Oh, and they're BMW tyres too so they're staggered. From the factory they're close in rolling radius but they're not identical.

@paulkelley86

Жыл бұрын

This may not be worth my time to respond, but I thought about it and other people reading will be severely misled by what you said. So here it is, for them, not for you: The title says AWD, not 4WD. So half of your argument is out the window. The specifics on tire circumference requirements all depend on the design of the system, and generally speaking most 4WD are part time 4WD with no center differential, so we will go with that design. Running around on dry pavement, you don't have to worry because you will be in 2WD, the rear driveshaft only spins, no issue with binding the gears in the transfer case. If you do enter 4WD, big issue with binding in the transfer case except you would only be in 4WD when traction is an issue. Because the tires have an easier time spinning, either the front or rear will slip a bit to equalize driveshaft speed. No issue, again because you are not on great traction dry roads. If you did however try to drive in 4WD on dry roads, you would have a major issue, try it out, but you have been warned. 4WD with center differential, totally different. This would be a "full time 4WD". I can tell you are European or something, and I am not familiar with all those models, so ill list a couple that we typically see in US... Hummer and Land Cruiser, not common vehicles. These have a center differential in the transfer case so they can run 4WD on dry pavement and not bind up on turns. It is very specific not to engage the center diff lock in good traction scenarios. That is because engaging it will make it function like a part time 4WD, there will be no allowance for front to rear driveshaft speed differential. Meaning again the tires have to be able to slip easily to compensate. If not, you are severely binding the transfer case and will cause failure. And Finally AWD, which is what this video is really about. Front to rear drive, commonly driveshafts but often may have a transaxle directly driving front ring gear with a PTO transfer case coming off to rear driveshaft etc. are drastically different. There are many designs, many of which use a clutch pack/viscous coupling between front and rear. So when there is a speed difference between front and rear, the clutches will slip to compensate. That is totally fine, since it has to do that around turns anyway. Clutches wear on these over time, the heat produced is pretty minimal. No big deal. The fluid will cool down on the next straight away anyway. The problem comes when you are going straight on the highway for miles and miles. If your tires are mismatched front to rear beyond the specification, there is a speed differential. This means there is clutch slipping on turns, but also on straights, all the time. It never cools. So it builds up heat over time and this ultimately leads to fluid breakdown and clutch burning.. ultimately transfer case/transaxle failure. So you and other people are taking information for one system and applying it to other systems that are not applicable. As for your request "you want to hear it from an engineer"... Well.. engineer's don't give speeches, they don't generally create helpful videos... they do however author service manuals. So if you want to speak with the engineer, read the service manual. I work with engineers on a fairly regular basis, it is very typical for them to give the information they feel is needed for service, but not all the information. Especially with Japanese manufacturers. So this information from me, came from the service manual, which they wrote, but it is interpreted a bit. Every AWD vehicle will have specifications for tire replacement somewhere, buried in the owners manual, service manual, or similar. It is often very difficult to extract the information you want. I found the 3% specification, and I shared/explained in a way hopefully people can understand. Clearly not you. But I would also like to agree with you on a point. There are millions of people replacing one tire at a time on AWD and not burning a transfer case or clutch pack. I agree with that. Exactly why is hard to say. Maybe the tires are within 3%. Maybe the engineer who specified 3% rounded down from 3.9%. Maybe they wanted to build in a buffer. Maybe its more of a "just in case". Kind of like oil change intervals. Change at 5k miles, and yet I saw brand new cars sold come back with 15k miles oil never changed and never skipped a beat. Some of it may not be an exact science and they picked a better safe than sorry approach. You all are welcome to make your own decisions, but if you are a little more educated and I helped translate for you, then great! Part time (selectable) 4WD, probably not an issue, just be mindful when you drive in 4 wheel drive there should be snow, ice, or mud where the tires meet the road to reduce stress on 4WD components. Full time 4WD (rare), same as part time except you should be good as long as you are not locking the center diff. If you lock the center diff, make sure there is snow, ice or mud traction. AWD hopefully you can find a spec in the manual, if not I gave you an example. Professional recommendation: do a set. I make nothing off it. If you are frugal like me.... do a single tire but same model tire. Or.... do whatever you want, there is a questionable amount of error allowed. All depends on what type of gambler you are.

@matthewnicholas6365

Жыл бұрын

@paulkelley86 BMW xdrive is described as full time awd with a rear bias of 60% It's not a complicated system like, say a land rover that can split power left/right &f front/rear, but it is awd all of the time. There have been cases when the xdrive transfer box has failed. There have also been cases when every part of a BMW has failed because they're BMWs and they can't make reliable cars. As you're not familiar with non-US models, a car that absolutely did shit the bed at any kind of difference in condition was 90s Vauxhall 4x4 models. Notably the Cavalier and Calibre (same car basically) Almost all of those, whilst still relatively new ended up being converted to FWD only. On the other hand, Ford had the 4x4 Sapphire Cosworth which was permanent 4wd, it came out at the end of the 80s and I've never heard of one failing. And that's a car where a lot of bad owners had them and worse, modded the hell out of them. Engines and gear boxes gave up before anything related to the 4wd system You are probably onto something with interchangeable use of phrases. But I think it's wider than that. I think people take experience with something like the Calibra, a car with a transfer box that absolutely will fail, and apply it to everything to this day. Don't take my comment as condescending to you, when I say I want to speak to an engineer, it's because I've never once heard an engineer involved in the design and release of a 4wd system ever talk about this issue. In the 80s and 90s when these systems began gaining traction (see what I did there?) They had fairly cheap tyres. It's over £1,000 for 4 these days. It's bad enough that a lot of shops won't fix a puncture on a run flat, let alone being faced with having to change all 4 because of one (if, say the last set are a few thousand miles old) It's not practical to sell cars with that flaw. And if it was recommended, the handbook and dealers would say so. And trust me, they don't.

@paulkelley86

Жыл бұрын

@@matthewnicholas6365 all good, we are on the same page. If it's got a split bias it has to have a clutch pack. I do agree there does not seem to be enough failure to support a typical shops hysteria about needing all 4. I attribute that to lack of information and poorly trained techs. Good luck out there!

@williamg8853

Жыл бұрын

@@paulkelley86 This is a good video and discussion, but you repeatedly say, to the effect, "There are millions of people replacing one tire at a time on AWD and not burning a transfer case or clutch pack. I agree with that. Exactly why is hard to say. Maybe the tires are within 3%. Maybe the engineer who specified 3% rounded down from 3.9%. Maybe they wanted to build in a buffer." it's simple. The difference between a fully worn and brand new passenger car tire is not close to 3%. I looked at the smallest BFG K02 (which has a 15/32nd new tread depth) and even a fully worn tire in its smallest size is just 3% smaller than new. Basically, if 3% is the rule, no combination of new and fully worn tires should cause driveline problems.

That’s why I would never own an awd vehicle

@lenderzconstable

Жыл бұрын

Never again. This situation is fucked!!!

If this is true, the car has a crappy design.

Bullshit!!!! No you do not.

Paul is there a way to figure this out with tread depth instead of 3%. I just bought a 2020 AWD Mazda 3 automatic non turbo. The rear tire looks to be newer than the other 3. Same brand and size. The front tires are both about 6.5/32nds. One rear is 7/32nds and the newer one is 8/32nds. So there is a 2.5/32nd different in the front than the back newer tire. Will this be a problem? Thanks

@paulkelley86

Жыл бұрын

Same brand and type I will bet no issue at all. In fact with that little tread difference I can guarantee it. Also like someone else had commented, engineers tend to build in a buffer so 3% is what they say but I wouldn't be surprised if a little over 3 caused nothing either.

@AceFaceDesigns

Жыл бұрын

@@paulkelley86 Thanks for the reply. I was stressing about it as I didn't want any issues. I had a mechanic friend look at it and he said it shouldn't be an issue but I wanted a 2nd opinion. I read that you can have up to a 3 or 4/32nd difference without issue but I wasn't sure.