4WD damaged by uneven tire wear

I damaged a tire on a set with about 25,000 miles of wear, and I replaced only the one that was damaged. The mechanic at the dealership told me that this can cause damage to the 4WD (2006 Subaru Outback). He says I must replace all four tires any time I need to replace one tire. He says it places uneven stress on the 4WD. Is this true?

look in your owners manual. what does it say.

i believe you have AWD. NOT 4wd. there is a huge difference. AWD WILL chew up your transmission and differential if you have different size tires.

yes, you need to replace ALL 4 tires on an AWD vehicle.

but, read your bible (the owners manual) it has alot of info in it.

It can, depending on the mechanism. You may be able to get the new tire shaved down to the same diameter as the old ones.


Dependent on your transmission (manual or automatic) and if you have limited slip or not and where the new tire currently is will dictate damage if any is done. Manual transmissions are more tolerant to different tires but the limited slip will burn out (ON ANY CAR AWD or FWD or RWD) if you have it and your new tire is there.

Your vehicle has all-wheel-drive. Not 4 wheel drive. And this means that not only does the vehicle have front and rear differentials like a 4 wheel drive, but it also has a center differential unlike 4 wheel drive. And this is where different tire diameters can cause damage. But guess what? They now have a service called tire shaving! This means that they take a new tire, and they shave the tread down to where it matches the diameter of the three tires that they recommend that you replace! And this service has been invented for those that drive all-wheel-drive vehicles!


Yes, the mechanic is correct. If you are still driving on the mis-matched tires, you may already have damaged the center differential. But, if you get the 3 additional matching tires that you need TODAY, you may be lucky enough to be able to avoid having to replace the center differential.

I would also suggest that you read your Owner’s Manual regarding this point, as well as for all of the other very valuable information contained in it. This is just one example of how failure to read the manual can be very expensive for a car owner.

Incidentally, if that center differential does fail, it will not be covered by the powertrain warranty, because your failure to do what is stated by the manual constitutes owner negligence and the dealership already knows that you mounted mis-matched tires.

Because of the all-wheel-drive, the maximum acceptable difference in tire circumference is 1/4 inch. After you divide by 2 x Pi, this corresponds to a difference in tread depth of a little more than 1/32 of an inch. Considering that new tread depth is 11/32 of an inch, the allowable mismatch isn’t very big.

After replacing two sets of half worn tires on our Subaru because one tire in the set had been destroyed, I checked with Subaru customer service. They confirmed the 1/4 inch limit. If you drive with mismatched tires, damage is inevitable and will cost far more to repair than a set of tires. This is one of the down sides to all wheel drive.