If it's both, then the speedometer gear in the transmission should be changed.

If it's only the speedometer that is off, there is a clock spring inside the instrument cluster that can be adjusted.



There is several ways of adjusting the speedometer (all involves removing the speedometer cluster).

-A speedometer shop can recalibrate it.
-If you have a drill with a known rpm and a piece of speedometer cable, you can calculate the mph and bench calibrate it.
-I did mine by comparing it to a known good speedometer in another car. It takes at least three people, and another car (one with cruise control is best). Remove the speedometer from your car, crack it open to be able to adjust it, have the mph face available for reference (make a mark on the case at a certain mph [like 55mph] to calibrate to), connect the speedometer cable (the third person will ride in your 122 and do the adjustment), get on a road that you can set the speed (like 55mph) on both cars for a good distance, and adjust and reconfirm until you get a constant reading matching the other car speed. The last thing is retack the clock spring down like the factor did (They use some greenish paint [should be able to still see it], I use some fingernail polish).

Before recalibrating my speedometer, it was off by +20% (reads fast) or so, but the odometer was only off by +3% or so. Now the speedometer is off by -3% (reads slow).

Paul