It starts on the ground at the rear wheels and works forward to the cable, and utimately to the generator.. There is a reduction gear in the diff and another on the mainshaft, in the back of the trans.
Not much can go wrong with a speedometer.Metal shavings / rust particles stuck to the magnet can be an issue, but then the needle is jumpy. It start by checking the rear end ratio to be sure & then changing the speedometer pinion gear if it were mine.