Check the accuracy of the odometer, that will tell you if you have the correct drive gear as the odometer is a direct drive from the drive gear.
The speed cup moves based on a magnetic field that can be adjusted in the Stewart Warner speedometer.
So if the odometer is correct, you have a calibration problem.
If the odometer if off then install the correct drive gear for your differential, then if the speed cup is off you can calibrate the speedometer to correct the indicated speed.
Here is a link to Gordon Fishers instructions to rebuild and calibrate a speedometer, it is for the SW oval speedometer, but will help get you through the round speedometer.
http://www.modelahouse.com/tech/spee...r/fischer.html