On several cars so far I've checked the speedo reading against a stopwatch-timed flying mile. I use the highway mile markers as a distance reference. I think I'm getitng time within a second. I assume the State of Michigan can measure distance within say 10 feet. At highway speeds I usually find the car speedo reading high by 3 mph, which is around 4% at 70 mph. I haven't seen one read low, only read high. Instrument cluster designers I've worked with have told me that car companies try to err in the directio of overreading, to help customers avoid traffic tickets.

Road fan