I posted this before but it disappeared:
The speedometer on these AD trucks does not have a stop external to the face. Therefore if you give the needle some gentle counterclockwise persuasion, it works against the internal stop. Also the needles are made of steel, or at least mine were. I have two "practice" speedometers that I honed my technique on.
For checking the head calibration I turned the input shaft at a constant rate of speed using a drill, for a measured amount of time. I then used the odometer readings to calculate what the speed was. My unit checked out perfectly.
Fred
