At a minimum you will need know exactly how many MPH at a given speed (say 55 MPH) your speedometer is off and the number of teeth on the driven gear.

If the speed difference is too great to change with the driven gears available, you will have to change the drive gear on the output shaft of the trans (requires removing the tail housing).

Some Chevy trucks used to use an external speedometer adapter that can also be used to correct the speedometer difference (they are marked on the side with the ratio).