True. The amount of torque generated within the speedometer can't really fall out of calibration. It's same principle as an AC induction motor, but kind of in reverse. More speed means more slip between the magnetic field and the rotor, which means more torque. I doubt the magnets or the rotor have changed significantly over time.
The response to the torque, and the subsequent needle deflection, would be dependent on the spring and friction. Corroded springs or pivot points are probably the biggest cause of speedometers getting out of sync with odometers. You may not be suffering from any of that, though.
If your odometer and speedometer don't agree, then the speedometer is already degraded. If you were to calibrate based on speedometer readings, I would fear that the speedometer would degrade even further and your accuracy would be fleeting. I would calibrate based on the odometer, then carefully clean/lube/refurbish the speedometer parts if it still seems inaccurate.
Last edited by pvfjr; 05/05/2016 9:49 PM.
