We repair and calibrate speedos. The calibration setting is marked on the back of the unit. It represents either the number of revolutions per minute at 60mph or the input frequency at 60mph (depending whether mechanical or electronic). When units are sent to us that is how we calibrate.
If we have the car and the speedo then we can plot a curve off our chassis dyno which will allow us to calculate the difference in the 60mph calculation. Most areas have someone measuring and certifying speed with a dyno - in the state of Florida police can not use radar from a moving car unless the speedo is certified every six months. So there are plenty of test stations. With such record any speedo shop should be able to do the calculation.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician