There is a little gear in the odometer assembly. A tooth has sheared. I think that it is a common problem. Mine did the same thing. Nobody would sell me this 10 cent gear. The dealership wanted to put in a new odometer plus a sticker that said that the mileage wasn't actual.

I found a shop in Austin that would rebuild the odometer for $100. I removed the cluster ( Bay13) and disassembled it ( lots of screws ). The odo unplugs from the circuitboard. I mailed off the odometer. When it came back I reversed the procedure. So, time $100 and a little skin was the cheapest fix that I could do.