I have repaired a couple of mechanical odometers/speedometers, 1 on a 1975 450SE and 1 on an early 70's 230(?). If yours isn't electric, read on.
In both cases there was a broken or loose gear on a splined shaft that needs to be replaced or repaired with epoxy. Take your instrument cluster out, then remove the speedometer part, and you'll quickly see the problem. Test the fix by chucking your drill onto the cable end of the speedo head and running it in reverse. Should read about 40mph, if I recall, but it's been years since I did this.
Do a search too, as I know I've read some posts on this subject.

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1986 560SL
2002 Toyota Camry
1993 Lexus