Uniform tires for AWD -- why?

I have two Subarus and an AWD Passat. Both manufacturers insist that all four tires be the same. It certainly increases the expense for a flat – especially if the other three are a discontinued type of tire. What is the reason? It would seem that it would make less of a difference for a car with independent drive to each wheel.

Each of your car has limited slip mechanisms that keep the wheels from spinning in slick conditions. But these mechanisms react to different-sized tires as if one was slipping, trying to lock up, which isn’t possible. This damages the mechanisms.

If you find yourself with needing a new tire with 3 old tires still in good shape, see if you can find a tire shop that does ‘shaving’ to get the new tire sized equal to the old ones. With 3 awd vehicles this will come in useful.

Open differentials are like water running down hill with the power going to least resistance. Because of differences in diameter or traction on a steady basis, you can imagine how imbalances the wear can be on those gears. Plus, different tread designs often have different tread depth. That sounded like a pretty poor explanation, but it’s the way I think of it. Slight differences always exist and modern systems are quite durable. But just about everything you do to vary the rolling diameter and traction between wheels does increase wear. All cars recomend that you keep tread designs and wear on all tires reasonably close, or at least they should. In that respect, your requirements should be no different from awd then 2wd cars. But, for those of us who like to do things on the cheap and stretch the safety envelope, we have these debates all the time.

In fact your drive is anything but “independent” to each wheel. If one tire is different it creates stress in the drive train and this stress can actually build up to a point where stuff snaps. It could be a case, a shaft, a joint, or a gear. AWD systems on dry pavement can tolerate only small differences in tire circumferences hence the warnings to match all four tires.

Yes, tires are expensive. Even more expensive is replacing and/or repairing AWD drive components.

Its because they want to ensure…positively… that each wheel spins at exactly the same RPM…it has to do with the AWD system…the center diff, front diff…rear diff…and they are correct. Dont test them on this point…Trust me

It has even been noted that the same size tire say 205/60/15…may actually differ in overall height or circumference between different manufacturers…Im sure it is by a small margin, but I have seen this info written somewhere along the way…I have no reason to doubt this…as tread depth may be a factor…whatev…just follow the AWD rule and you wont be on here weeping about expensive diff fixes…LOL

Many AWD vehicles, including Subaru, use a wet clutch in a very viscous liquid to keep the front and rear axles turning the same speed. In a turn, the front must turn faster than the rear and the clutches slip. as designed. If one tire is considerably different in size one axle will turn at a different speed continuously, even when traveling straight ahead. Doing so stresses the clutch pack, wears the clutches, heats and degrades the viscous fluid and keeps the entire drive train under unnecessary stress, i.e., wear.It is important to keep the tire diameters uniform within reason. Often tire dealers and shops get extremely anal on the issue in an effort to sell more tires and keep an “ace in the hole” regarding explaining problems.

Yep, its definitely important in the AWD Subies… The outer wheel will always need to spin a little faster than the wheel on the inside of the turn…but that is a job for the front diff…no? I guess that extra speed could be transferred to the viscous coupling somehow also…

The point being…they all need to be identical… No one disagrees with this methinks…thats cause we know whats up…

Thanks to all. The explanations grounded in mechanics make a lot more sense than the tire salesman’s “because I said so!”

Consumers should analyze whether or not they really NEED AWD or can they live without that VERY expensive feature. Most people find that 95% of their driving is on clear, dry roads where AWD offers little benefit and adds up to significantly increased operating cost…

Evidently AWD cars shouldn’t go around corners because the outside wheels will rotate faster than the inside wheels. I’d like to hear from someone who has poo poo’d this recommendation and driven his car for thousands of miles with slightly different tire diameters. Me thinks this in another one of those “facts” that everyone knows, except no one has put it to a test.

OK, I will tell you the opposite.

I have been involved in several situations where tires of different diameters have been applied to AWD vehicles and the center coupling has failed. Right now I’m dealing with a 2008 Cadillac - and boy, is this going to be expensive!!

But I am sure you will find some guy who will either swear the problem wasn’t the tire diameters - or some guy who will say they got away with it. That’s great, but the fact remains, there have been plenty of instances where center couplings failed that can be traced to different tire diameters.

And just to be clear, it isn’t that going around a corner isn’t a problem, it’s the constant difference that’s the problem. A few second difference doesn’t cause enough heat or wear to cause the coupling to fail - but thousands of miles can!

Going around a corner is momentary…Mismatched tires are a constant load on the center differential which in many of these cars is a fragile gearbox…You are probably right, it’s not that big a deal, but when car dealerships started laying off warranty claims for failed AWD systems on mismatched tires back to the tire stores, another legend was born and tire sales got a nice boost…You go into a tire store with a AWD vehicle today and it’s four new tires or nothing…

lars46, I suggest you take a moment to be un-troll-like and thoroughly read the insightful explanations above. You might learn something.

Edit: Keep in mind you take corners a lot slower than you drive in a straight line on the highway.

Mismatched tires do cause wear and tear on AWD vehicles, including destruction of the ring and pinion gears.
As CapricRacer stated, a few seconds may not matter but many miles does matter.

We used to get Subarus in for problems like this and the common denominator was mismatched tires. In one case a Subaru owner (rural mail carrier) came in with a vehicle that 4 different sizes and brands of tires on it.
The car was a manual transmission and was jumping out of gear because of the tire mismatch. His solution was to forcibly keep it in gear and so not only was the ring/pinion damaged but the shift fork and synchronizer sleeve was prematurely worn out. (New transmission only cure.)

Unless Subaru has changed it. their front and rear differentials actually have a very tiny ratio difference although I do not remember exactly what it is; something like 1/100 of a revolution variation between front and rear.
That was by design though; larger differences due to tire oddities are not part of the design.

Keep in mind, though we do a good job talking about the importance of keeping the rolling diameter of awd car tires the same, in practice, my self included, we sometimes get lax and omit little things like making sure tire pressures are correct and spreading the loads front to back and side to side on these cars. These factors if great enough, have as much if not more affect on rolling diameter then “differences in tire wear”.

For safety, we should be doing this to ALL cars. So really, there should be little difference in practice of proper tire selection and maintenance for all cars and trucks, regardless of the drive train.


AWD vehicles are designed for different speeds at each tire as a vehicle corners. There is certain differential in speed allowed. However adding what is considered a considerable differential in size (AWD maker specific) only stresses the system the most when it is turning the sharpest. Because the differential in speed is now increased from inside and outside tire and likely outside the designed safe range. You also constantly are slipping the differentials driving in a straighter line and the slippage turns to one thing “heat” in turn excess wear.

Whitey: If I disagree with the common idea, that makes me a troll? Disagree. By the way, we have lots of bends in highways here in Western Pa that are taken the same speed as the straights.

If these AWD cars are really as finicky as you all claim, then I say this is a bad design. I’ve never heard such complaints about 2 WD cars.

I couldn’t reply earlier because the web site wouldn’t let me on for several days.

lars46, I don’t think disagreement is necessarily troll-like behavior. What I thought was troll-like was that instead of posing a question, or simply offering a contradictory opinion, you started with confrontational sarcasm (often my stock-and-trade, I must admit), and then, you simply said everyone else in this conversation is wrong and you are right, but you didn’t back-up your position with any logic, facts, or even any fallacious reasoning. You simply touched the discussion with a needle, as the ancient Romans would say, but didn’t contribute anything useful or profound.

Lars…WHAT are you arguing about? Do you not believe it when we…MECHANICS tell you that you will harm your center differential…or your viscous coupling in an AWD vehicle with mismatched tire sizes? If you DO argue this point it only declares your lack of proper information about how these systems work.

IT IS A FACT…that you should not have different sized tires…it WILL harm the AWD components. Going around a corner is well within the design parameters of the center diff (with matched tire sizes)…for the temporary wheel speed difference… WHen you have a mismatched set…this puts constant strain on the center diffs and viscous coupling…CONSTANTLY…which will do the harm…the temporary offset in wheel rpm wont hurt it at all as we arent talking about the constant load of the different sized tires. When you have a mismatched set…the diffs and center coupling are constantly trying to switch power between one wheel or the other…and I mean it will try INFINITELY…this torque vectoring will be there in an unending fashion…basically it will add to the power of X more strain on the system…its a mathematical fact Sir…sorry.

Just trust us…please. To argue this point is useless as it is a simple fact. We arent tire salesmen on here…just very experienced wrenches trying to help people out.

Also your 2wd reference is lacking…because you are talking about the same exact thing albeit you are reduced down to one differential…now if you had 2 different sized tires on your rwd axle (which I’m sure you dont think would be the right thing to do)…you then bring up the same argument…you will be constantly taxing that one rear diff…NOW multiply this by a center diff and add another front axle with another diff in the picture…and the issue compounds…Because the issue then bleeds into front and rear torque division as well as left to right… You don’t see this? Should I get my trans-axle buddy on here to get really technical? Ooooh…lol…I think not…we addressed this adequately In my opinion. If you still disagree…crack open a few books on the subject of how differentials deal with torque vectoring from left to right in a turn… If you read thoroughly you will be on board with the rest of the guys…They know their stuff as well. I am just agreeing with them and wondering where you may be getting confused… We arent fighting here as there is no fight… Its black and white.

Finicky is a relative term. If three tires are 96" circumference and one is 95" the clutch must be forced to slip 10 revolutions per mile. At 70 mph that 700 revolutions per minute. That torque divider would be smoking after an hour of that. But of course, my figures are nominal. I’m sure someone will get out their Post Versa-Trig and fill in to the 3d decimal point.