Originally Posted by rodybmw

What I don't understand is why are both the 18" and the 19" tire size 255/55. This means that the 19" setup increases the overall diameter of the tire by 1", instead of keeping the diameter the same as the 18" setup by using a lower profile tire (for example 255/50 or 255/45).

When BMW offers bigger diameter wheels on other models they choose smaller profiles to keep the overall diameter close to the standard tires (as they should) so that the speedo, as well as all the electronic monitoring and safety systems would work correctly. However, if these sizes shown above are correct it would mean that there would be a big difference between the base models and sport models in diameter, which can throw off the speedo and other sensors.

Maybe BMW has a "loose" calibration for the speedo and other monitoring equipment that are set to somewhere in between the two different diameters???