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.