[olddog]

If a speedometer is reading wrong because the gear ratio in the trany cable was not correct, I would expect the error to be a percentage. That is if it was reading 60 mph at a true 40 mph it is reading 150% of actual, therefore I would expect it to read 30 mph at a true 20 mph.

Here is what I got with a GPS.

10 spedo - 10 GPS
20 spedo - 23 GPS
30 spedo - 37 GPS
40 spedo - 47 GPS - local police radar 46
50 spedo - 57 GPS
60 spedo - 67 GPS
70 spedo - 77 GPS

So from 30 mph to 70 mph on my speedometer the actual speed is always 7 mph higher. The error is a zero shift problem not a span problem. The only way I am aware of correcting a speedometer is to change the gear in the trany at the cable connection. Is there a way to adjust a zero shift in a speedometer?