When I got the car, the odometer didn't work and the speedo, when hooked up, would make this grinding sound and the speedo needle would jump all over the place. I recently bought a working mechanical speedo from a '74 914 so I could swap out the worn out gears.
So here is the 914 speedo:
...and my 912 speedo:
SURPRISE!! They are not the same at all! I stuck my power drill on there to see where the gears are that are failing (FYI the speedo turns when the drill is in REVERSE). The grinding sound is coming from the first worm gear, which isn't turning. So I decided to take that part apart:
I took a look inside and all the grease was dried up, so I just oiled her up and popped the back on again. (In reality it's not that easy. That big wheel is a magnet and it pulls the housing towards it when you try to put it back on. Also the little pin is hard to get back in to the little hole on the other side.. but I finally did it, although it took many, many tries..) Hooked my drill back up.... and it works!!!