Originally Posted by jphughan
OP do you have 18" or 19" wheels? I'm guessing 19 by the pic in your sig, but in addition to the fact that BMW does indeed calibrate their speedos to display slightly faster than reality (though this can be disabled for the digital speedo via coding), wheel size makes a difference too. I don't know whether the factory adjusts the speedo calibration based on the wheel diameter you ordered, but if they only calibrate for one size, it would be for 19" to avoid risk of the speedo displaying too low when the car has 18s -- and if that's true then people who have 18" wheels would have their speedos showing faster by an even greater margin than those with 19". Of course even if the factory does calibrate differently for the different wheel sizes, if you switch sizes aftermarket all bets are off. Relative to the stock calibration setup, larger wheels will make you go faster than indicated, smaller wheels (like people who switch to 18s on the track) will make you go slower than indicated.
most people are sticking to similar outside diameter tires when switching. my 18 set up is .1 inch different than my 19 inch set.