Quote Originally Posted by nike001 View Post
Since you're going with a sidewall thats a bit bigger than stock spec, I'm unsure if this would affect your speedo or not.

My contribution to this thread is that today I saw a ZHP with 225 all around on stock spec rims (8"F & 8.5"R) and the rears looked a bit stretched. Definitely not noticeable unless you're looking for it. If anyone is in my area, I'll gladly trade you my 8" fronts for your 8.5" rears. I've been looking to run 225 squared.
BMW's get their speed from the wheel speed sensors, it matters not how tall of a tire you use. Same reason why a diff swap doesn't throw off your speedo, in old cars that are controlled by a gear in the trans it would, but not the E46. But interestingly standing side by side the 235/40 is almost the exact same height overall as the 255/35. I'm talking millimeters in difference, if any at all.