I work for the world's largest aotumotive parts manufacturer, and one of the many things I've built are speedo's.
The higher the speed, the more likely it will be off.
It's sort of an exponential thing, A 1 MPH error at 50 could end up as a 10 MPH error at 100.

Some will be perfect, most will be real close, and a few will be way off at anything over 100 MPH.

Matt