As "JackA" noted, pull it apart, clean off the old grease.. That's the "starting point"..
Generally speaking, the old grease starts to solidify to a point the Odemeter/Speedometer "drive gears"
can no longer turn against stiff resistance.. Then the (often) steel drive gear just chews-thru the phenolic
I repaired both the Tach and Speedo in my 1953 Jaguar XK120 by taking the units apart, cleaning everything,
regreasing (with a lighter weight grease) and reassembling.. That was some years ago and they still work..
The tough part is getting the "speedometer" to actually READ correctly!!
Hope this helps, some..
Charles 1974 FJ40.
There seems to be a lot of questions about Clusters, Gauges, Speedo and Odo meters, so I thought why not set up a page as a reference guide / database / knowledge center? Over time I collected a lot of pictures. I created a Gallery with all the photos I have, which you can see here...
There are some other one I could point you toward, but this helped me big time also...
Many here have seen or experienced the symptoms of the odometer not turning over properly. Usually the minor (tenths) wheel still turns over correctly but it does not increment the next digit. The most common cause for this is the lube on the odometer becoming old and sticky. My 84 BJ42 had this...
also here is the YouTube vid of guy taking apart and refurbing his: