Speedometers are seldom very accurate. I have a chuckle sometimes about the people obsessing about tire size differences and speedo accuracy. Most read faster than actual speed to some degree. I rented a new Jetta last month, speedo was almost 10% optimistic. GPS is the easiest way to check, although you can be quite accurate with mile markers and a watch/stopwatch.

Also, odometer readings are not always directly related to the speedo. One could be more accurate than the other. The odometer on the Jetta (above) was pretty much right on.

--
67 144, 86 740T, 91 945SE, BMW R69S, R60/2