I've been through a lot of Volvo odometers in the past couple of years. The primary problem seems to be the small drive gear where it takes off from the speedometer shaft. The little gear seems to wear out before anything else does.

Sometimes you can pull the little pin that holds the gear in place on its shaft and turn the gear end-for-end and have enough un-worn teeth to engage and keep the odometer working.

Probably the best solution is to take the whole speedometer or even just the worn gear to a speedometer shop and have the gear replaced. Nearly every town of any size has a shop that specializes in speedometer work for commercial vehicles like taxis or delivery trucks where the speedometers must be corrected and certified. They carry every imaginable part.

Good luck!

Steve