A while ago I compared three different GPS units against each other in a car...all three were within reading error of each other. Since then I have checked several speedometers against GPS and found that my Integrale over read by about 6%, the Fiesta about the same, the M3W was a full 10% over till I re-calibrated it to about 1% over and the two Hondas I've had were both bang on. I'm very surprised at any vehicle speedometers under reading compared to GPS: out in the open on a straight road and steady speed I would be extremely surprised if any GPS isn't very accurate indeed and that's without going to the more exotic and expensive units that are used for official testing of such things.
