In this video I find the problem with the odometer on my 1989 Chevrolet K1500. It turned out to be a relatively simple fix. Over the years, the plastic gear became brittle and cracked.

I later ordered a new gear.
Dorman part number: 924-396

You can get them on eBay. But search around. You may be able to find them a few bucks cheaper. I just ordered what was quickest.
https://ebay.us/hsRCOx


Music From Bensound.com
Song Title: The Lounge