If you go follow this link, it will show you what is most likely the problem with your odometer.
http://dieselgiant.com/repairyourodometer.htm
I fixed mine before that link ever existed, so therefor did not take it apart quite as much as you see in the pictorial. If you can find the gear that is slipping (show in some of the last few pics), then you can most likely fix yours without so much disassembly.
While you are in there, put in fresh lightbulbs, and clean up the white plastic parts that are supposed to help reflect light (I painted mine with model paint).
Good luck - let us know if that worked for you.
-Larry