[Inkedfireman]

Thanks to everyone for the handy RPM/tire size guide.

For anyone dialing in their speedo to new tires via reprogramming, the most accurate way of knowing the circumference of YOUR tires at the pressure YOU run so that you can use the supplied formula is to mark the sidewall and make a mark on the ground at the mark. Now, drive the car forward until the mark comes around again, and mark the ground. The distance between the marks is the TRUE rolling circumference. If you do it ten revolutions and divide the result by ten, it should be even more accurate. Tread thickness, the 'squirminess' of the rubber, the pressure you run, etc, can all affect the rolling circumference.

On an unrelated note, I've found out that 205/40-17's just can't seem to survive Chicago potholes. I've dented two rims and seem to have broken a cord/belt internally in one of my Ecsta ASX's, so I'm moving up to 215/45's. I really wanted 225/45's, as I like how they look on WRX's, but it seems they probably would rub from what I've read on here.

Denny in Aurora IL