The calibration is in the speedo head. I think the 83 SL has an electronic speedo (the 126 body with that trans does but the 123 doesn't) but in either case the calibration is in the speedo head.
We do these calibrations in our electronics shop and I am only partially aware how its done. On electronic speedos we use a number (K=xxxx) on the back of the speedo to determine what frequency signal to apply for 60mph. With the speedo running at that speed we place a new needle on the shaft and reverify both at sixty and at half frequency for 30mph or double frequency for 120mph.
Compensations for tire size or improper differential ratio are done only if we have the car and run it on our dyno to get a speed profile to alter the factory calibration.
[This message has been edited by stevebfl (edited 03-18-2000).]