Lennart (our forum administrator) runs 225/55-R17 on his car to fix the speedometer reading, but I have heard that it is possible to calibrate this using the diagnostic computer. I don't know the details, though.
/Anders
This is possible on all cars with ABS 5.3 with speed information, that means most 99 and all beyond.
Log on the ABS computer with NXR or CLIP - enter config and change the speed index according to tire size (5-6 different options). If none of the options is exactly the size on the car you need to calculate the closest match. This is a 2 minutes job with diagnostic tool, since i have 17" summer and 15" winter i reprogram this each season on my car - but thats because i already have the tools to do it.