Originally Posted by Schtum
Sorry Mac but I think you'll find this is incorrect. The computer takes into account the difference in rotation speeds between the front and rear wheels and uses this information to make decisions about ABS operation. So if you fit different sized wheels you have to re-program the ECU to take account of this, otherwise you bugger up the ABS operation.
Not sure where I read it... but one of the BMW techs around here said that when you first start moving, it is sort of an ABS calibration period. At this time, it figures the difference in wheel speeds for different size wheels. Also, several people have done wheel size conversions without reprogramming anything and did not have any adverse effects.