speedometers

[imscience@juno.com]

My '80 LE MGB speedo has both KPH and MPH on the dial.  Never gave it a
thought.
However, it reads consistently 10% high, based on mile markers and the
portable police radar speed displays (LOVE those things! great for
competitions!)  Anybody know how to calibrate a speedometer? (yes,
everything like wheels and tyres are stock sizes)  I know, I know, I
don't need to if I know the error, but it still bugs me.