Speedo works,odometer does not.any ideas.thx - 1983 Porsche 944

This is a straightforward fix. Anyone with small phillips and slotted screwdrivers and who can read can fix it. The part costs about $30.00 from Zims in Dallas. The replacement gear for the one which is broken comes with well-written instructions. Just follow them. Good luck!
The new gear is made from a tougher plastic than the original but still do not reset the tripmeter while moving.

I asked the porsche guy here, apparently there is a little plastic doohickey in the older cars that if you push the reset odometer button while the car is moving, the odometer breaks, only repair is to replace whole unit.