Here is a dialogue I found
I've got an odometer that is not working at all from my '72 '02. I've taken the speedometer unit apart and have found what seems to be the problem- a very small cast pot-metal gear seems to become detached from the steel shaft that should turn it. This gear is at the fast-turning end of the regular odometer and would activate both the "ones" mile counter wheel and the trip odometer if it were still turning with the shaft it's on. The shaft is smooth and the method of assembly seems to indicate that the whole "stack" of counter wheels for the odometer is assembled on the shaft, inside a bracket that is used as the end bearings of the shaft ("U shaped" with all of the counter wheels inside the "U"). Since both the inside
of the cast gear and the outside of the shaft are smooth, I can only guess how they were attached in the past- Loctite? There would have been no obvious method of attachment except some sort of glue, but no trace of the glue remains. I also don't quite know how the glue would have been inserted after the assemble was made. OR, perhaps the gear was made with an undersize hole and pressed on during assembly, in which case I might have seen some longitudinal "teeth" in the shaft in the proper area instead of the smooth surface on the shaft.
I've repaired speedos before and the units just seem to be pressurized although it's not impossible some kind of adhesive inside the cast wheel/shaft was used.
you've already determined that the shaft cast wheel/gear has come apart. This is typically what causes speedos in general to stop working. Looking at the back of the speedo gauge you'll see where the cast wheel shaft gets the speedo cable inserted. Usually with a few taps on the back of the wheel you can get the cast wheel and gear back in place and your miles and trip will start working again. Looking from the back of it, the two should be perfectly flush and it should not be sticking out at the outermost tip. If there is any play or it is not flush and sticking out tap it into place.
I didn't catch whether it was just the top mileage counter or the bottom odometer that was failing or both. If neither work the above is most likely solve your problem.
If one of the two works then your problem is elsewhere. The problem then will lie in front of the cast wheel around where the top and bottom gears synchronize which in that case you will have to take it apart and see.