Correct tire size question

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Originally Posted by KRAM36

I don't have 30 min to do anything.

You made the false claim. Two of you examples were not 1" different.

Do you have a valid reason not to try and match the front and rear tire rotation speed best as possible on a street car?

Yes, glad you asked. There is a very valid reason. On 987,981, and most 997 cars the PSM and ABS are precisely calibrated to stock tire stagger in diameter (typically 1" differential). Put the same diameter tire on these cars and the PSM overreacts in corners applying the rear brakes and cutting the throttle. It can also confuse ABS creating an "ice pedal" condition under heavy braking causing a crash.

Getting the factory tire diameter stagger is very important in Porsche cars after 2005 so that PSM and ABS function properly. Mounting the same diameter tire on all cars without consideration of factory designed stagger is foolish and dangerous. The 986 has a much simpler PSM and ABS that do not require precise matching of tire diameter. The OP can happily run the tires he is on with little or no effect. If he does change the rear to a 265/35/18 he will notice a bit better acceleration and his speedo will again be off by 4-6mph.