I second the Walrus' comment, but would add that you can, of course, check the speedometer with a stopwatch against a measured mile, or by having someone with a known accurate speedometer follow you on a quiet road and flashing lights at pre-agreed speeds. I know that you can adjust the ribbon type speedometers by changing the tension on the spring at the left of the drum - I don't know about the dial type, if that's what you have - but rather than going through that exercise, I would note the deviations and make mental adjustments as I drive. That's a lot easier.
Bob S.