I have 17's with 205/40's. My speedometer is off by a few. Like 3-4. But if I am going 45, it is on49-50 according to the radar signs. 45's on and it was accurate. 215-45-17 is the right size. but with 215-45's and sportlines, the rear scrubbed the fenderwells. even after rolling them. So, I have prokits now, and my next tires will be 215-45 again.