[patrickt]

It is always either:

1) the driven gear in the transmission;
2) a crack in the cable or debris in the cable causing friction;
3) a kink in the cable from too tight a curve;
4) the length of the inner cable going in to the reverser is too long;
5) the length of the drive shaft between the reverser and the speedometer is too long;
6) binding within the reverser from a lack of lubrication or malfunction;
7) the speedometer gears binding.

If the speedometer has ever worked right, then it is not going to be 4 or 5. If no changes have been made to the cable routing, then it's not going to be 3. You can diagnose 2, 3, 6 and 7 by reaching under the dash, disconnecting the cable and reverser and then spinning the cable with your fingers and spinning the speedometer by hand with the short driven shaft and then spinning the reverser by hand with the shaft. Spotting a problem with 1 requires visually inspecting the gear itself.