Originally Posted by Z K
You have 235/40/18 all around!? That is way smaller in diameter than the front or rear stock size of the car. You get less ground clearance, higher rpms for the same speed, and just plain looks weird. The speed sensor is attached to the transmission which reads rear wheel speed. You should be running 245/40R18 or 235/45R18 all around if you want something closer to stock sizing.
235/40R18 all around is shorter, but it is the size that BMW supplies with their winter package, so it's certainly not an unreasonable choice.