It is very likely that your speedometer reads high. In the old days, before computers (
) I used run a measured mile at an indicated 60mph and see how long it took (it should take 1 minute) and the two rarely matched over the years and cars I did the test on. I beleive manufacturers would rather have you think you are going faster than you really are for "safety" reasons. I also think most manufacturers account for a small +/- percentage to take into account the use of different sized tires (like taller snow tires) for instance.