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I put 4 new tires on a Subaru Forester @ 35,000 miles. Car now has 60,000. I noticed a small bulge on the sidewall of the front tire and had it replaced with the unused, full-sized spare. The spare is the same size Bridgestone but a different model which I know is not the right thing to do–but does it really matter? I have a habit of scarping curbs when I parallel park, and it is beginning to cost a lot! Can I get away with the spare as a permanent replacement? (Please don’t tell me I need to improve my parallel parking skills. I have no such troubles parking my other car, a Jaguar XK8, which is a good thing given how expensive THOSE tires are!)

Pull out your owner’s manual. You really have two issues to deal with.

  1. You need to take a steel tape and do a measurement of all four tires, per the Subaru owner’s manual instructions and see if you fall within their tolerance. Subaru has a max tolerance before you start screwing up the AWD system, which is not cheap to repair.

  2. Subaru also recommends that the same model and similar tread type be used, but stop at requiring it, as I recall. Follow what is in the owner’s manual for the best solution.

Does your Forester have an automatic transmission or a manual?

As jayhawkroy explained, Subarus are very sensitive to differences in tire circumference. I would definitely check the owner’s manual on this one, because if the AWD transfer system is damaged, which it can be by different size tires, it can cost some money.

I paid about $750 to replace the transfer valve and clutch pack in my Subaru’s automatic transmission a few years ago. I’ve learned to make sure the tires are all evenly matched.

Manual transmissions are much more tolerant to differences in tires but still risk. Less risky if the odd tire ends up on rear with open diff(lower end Subaru models).

See for tires info.

Learn to drive better or buy cheap tires likely in sets of 4.

Since the other three tires have 25,000 miles of wear on them, the unused spare is definitely NOT properly matched to the other three, and that is without even considering the issue of a different model tire.

If you drive more than ~100 miles with the mismatched tires, the extra wear on the car’s center differential as a result of the mismatched tires will result in early failure of that part and it will cost…perhaps as much as $1,000 to replace it.

If you read the relevant section of your Owner’s Manual, it will caution you against driving with mismatched tires, but it will not go into the amount of detail that we all are providing for you. If you decide to continue to drive the car in this manner after you have read our comments and after you have read the Owner’s Manual, then you may want to research the concept behind the expression, “Penny-wise, dollar-foolish”.