Hi lonestarsally,
Hmmmm, so he installed the speedometer onto a motor to run it for 3 months to calibrate it?
Running the speedometer to get the calibration is akin to getting an old speedometer as it has been operating for so long.
I have seen people doing it for less than 1/2 a day. It was done electronically.
Something looks fishy.
I have no suggestions or comments on what you should do but the guy doing the job now is definitely not up to the mark.
Btw before changing the speedometer, was it confrmed that the speed sensor was not faulty?