I could be wrong, but if the wheel size is changed after the fact wont the speedometer on the car be slightly off? As an example: Going from 19 to 21 wheels the speedometer might show going 65MPH but the car is actually going about 72mph (=speeding tickets if not aware of this)? Or going from 21 to 19' the car might show going 65mph but it is actually going about 58mph, since the speedometer is calibrated for the specific rotation speed of the wheel size when it left the factory.. On a normal car the little gear would need to be changed when the wheel size is changed, but on this car it MIGHT just be a software adjustment letting the car know its current wheel size?..