All Wheel Drive and new tires

Recently my Audi A4 Quattro got a flat and when I when to replace the one tire I was told I needed to replace all four as it would “screw” up the balance in the transmission. Naturally I thought this to be a scam so I went elsewhere and bought my tire. But it naggeed at me so I thought I would ask if anyone knew what the “real deal” is?

It’s entirely possible. Some AWD systems have very little tolerance for differing diameters of tires. We’re talking in tenths of inches here.

The key to whether or not this will cause very expensive damage to the AWD system (and possibly to the transmission) is the difference in diameter between the 3 “old” tires and the new one.

What can you tell us about the number of miles driven on the “old” tires?
What is the tread depth of the old tires?

Also…the answer that you seek may very well be explained in detail in the Owner’s Manual.
In fact, I would be very surprised if Audi did not address the issue of differing tire diameters on their AWD vehicles.

Generally speaking, the tires should be all the same size, tread pattern and no more then about 2/32 inch difference in tread depth. This is assuming the tires are the same model. If you don’t do this, it doesn’t guarantee failure. It increases the wear of the differential gears and generates more heat. What does that mean ? Overtime,these and other components in the differential will wear faster. If you trade within 50 to 70 k miles, it may not mean a thing, as though you had oil changes at longer intervals then called for. Rest assured though, some one later down the road has increased chances of making an early repair. There are lots of things you can do to increase the wear of these gears…this is definitely one of them.

Read your owner’s manual It will tell you if your car must have matching diameter tires. Most likely it will say that your tires must match in diameter to within a small tolerance.

What the tire dealers do not tell you is that one new tire can be “shaved” to match the diameter of the other three tires. If your manual does say that tire diameters must match, call around and find a tire dealer that will “shave” your new tire to match the diameter of the other three.

You can read more here:

How many miles are on the other tires?

The coupling between the front wheel drive and the rear wheel drive is usually a set of clutch plates that are allowed some slippage. If the tire circumferences are not pretty close, you add a lot of wear to these clutches.

You can have the same problem if you don’t rotate your tires on a schedule. If the tires at one end of the car wear down faster than the ones at the other end, and you don’t rotate according to the schedule, then you cause excessive wear also.

There is some tolerance for differences in wear so as long as the new tire is within the tolerance, or should I say the three old tires are within the tolerance of the new one, you should be OK. You may want to put it on the end of the car that wears out the tires the quickest though, usually the front but not always.