Thanks for all the input. If anyone knows where to find the previous discussion please let me know. My concern is that I have a BMW, Honda and SM all w varying accuracy, the SM being the least accurate - about 7-8 mph too fast according to GPS. Up to 5 is easy to live with, but this is about 15% off reality. Of course the BMW is spot on, just to cause trouble! Also what Benny said about the speedo vs. the odometer makes sense. Thanks for the explanation.
