93 Spedo way off sometimes

My friend just bought a 1993 and his speedometer works good. Sometimes when on the highway, it's as much as 20 mph off. Other times it's right on.
I was wondering how it works in this year/model? Is this a trouble spot with this car? What could be the problem? Thank you
Chris.

A vehicle speed sensor may be the fault (if the gauge is electrically controlled) also does he have factory rims on this car or are they larger/ smaller than what came on it originally? That will throw speedometers off as well