The odometer fails because a press fit pot metal gear loses its grip on the shaft. A small amount of dirt or expansion of the metal can help it regain its grip. The real test comes when it has to turn all the digits at one time.

The above is from a previous post.

My 84 300 CD is 500 miles short of 300,000 (and has been for the last week or so when I finally realized that it wasn't getting any closer). Assuming this is the problem, since the speedo works fine and the 299 numbers are half turned, how easy is the fix from a time perspective? If its a real pain, I'll just leave it and change my oil every 5 weeks instead, which historically usually corresponds to 3000 miles.

Thanks

MikeL