I take it the speedometer is working. If not, it is ninety-per-cent certain that it is the speedometer cable. Not an expensive fix, even with a Volvo part.

If the speedometer is working, then the easy fix is not available. Have you tried fiddling with the tripmeter? Perhaps the shaft is not disengaging fully or it is in some halfway position. I would certainly make sure that the shaft is not messing up the odometer in any way.

Next on the list would be the removal of the speedometer unit. There is probably a worn-out gear between the odometer and the speedometer. You could probably pick up a speedometer at a reasonable price in a junkyard and either replace the whole unit or fix your own [if the replacement is different in some way from yours].

They are usually pretty reliable so there won't be a great demand for them from the junkyard.

Bob