I recently noticed that my BMW speedometer is not in line with my Garmin Speedometer, they seem to get further off the faster I go. I spoke with my local bmw mechanic about this and he told me it was because I have larger wheels than the stock car had; but didn't offer any suggestions on how to recalibrate the factory speedometer. What can I do to fix this and get accuate speedometer readings? I was thinking I might get a diff. gps unit and have it slaved to wheel spin but that still doesn't fix my instrument panel readings. Any ideas?