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To adjust the calibration, you need to balance the strength of the rotating magnet and the hair spring on top of the speed cup. There is a slide lever on the bottom side of the magnet which will increase or decrease the pull on the speed cup. In order to get the unit in calibration, you need something to turn the speedometer at a known rpm. 1000 rpm is equal to 60 mph. You can make the speedometer read correct at any one speed by just adjusting the hairspring, but it will not be accurate over a wide range unless you adjust both the magnet and the hairspring. If the magnetic pull cannot be adjusted high enough, then the magnet will need to be recharged.
All this is assuming that the odometer is reading correctly, meaning that the tire size and rear end ratio is correct.
Mark's Model A Service
www.marksspeedometer.com