First off stock speedos are not very accurate, especially at speeds above 100 mph

Using my GPS locked to a 13ft accuracy, my Saturn is accurate to 0.5mph up to speeds of 100mph. I've never gotten it above that yet with the Garmin.

Rob's Viggen was a bit off, and my motorcycle is about 5mph off at 75mph. GPS with a decent accuracy tends to be a good tester of speeds, as long as the speed is stable. Rapid changes are hard to pick up due to the refresh time of units.