The speedo was stripped, cleaned and inspected.
That said, magnet degradation and loss of spring tension could not be measured easily. There is also needle positioning.
The last would be a constant offset while the first two will show up as a percentage variation.
I found a local road with a 2 mile post marking. I will run that against the odometer to get an over ground error. There is no way that can be used for a speed reference.
Then I will try shorter runs at slow and fast speeds against the GPS and see if I can get enough consistency in the readings to determine if the speedometer tracks the odometer.
If I have to choose, the correct speed is more important than the correct mileage.
