Hi Red,
Do you mean you have disassembled the speedometer or just removed it from the dash plate? The screw on the back of the speedometer case just holds the inner mechanism to the case and doesn't adjust anything.
It is possible to adjust the speed slightly, refer the the instructions here:
http://www.modelahouse.com/tech/spee...r/fischer.html
Also, if the mileage reading is incorrect per the odometer, as well as the speed, the problem may be an incorrect drive gear.