I found this thread
Electric Speedometer Repair

I also did some quick reading around the inter-webs, and it seems that if your odometer is working but not the speedometer its the speedometer gage its self and you should replace the whole thing, best bet for that is the junk yard. if your speedometer works but the Odometer is out, thats the mechanical gears that can be replaced on the back of the speedometer. if both are not working then look to the sending unit.

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