[mikepellegrini]

I fixed mine (an early 944, which I think is the same). It was actually pretty easy.

I had to take my speedometer apart to fix the odometer (a gear was slipping). As part of the disassembly, you have to take the needle off.

I have lower profile tires than normal on my car, so the speedometer was off already. I put the speedometer back in, and then drove at a range of speeds - all while I checked the actual speed against a GPS.

Doing this, I came up with an average amount of error (in my case, it read about 10 MPH higher at 70-80 MPH than it actually was).

So then I took the speedometer out again, and simply took the needle off, and re-mounted the needle so it read 10 MPH less at zero (zero is now actually about -10 MPH). There's a pin that works as a stop at zero - you have to break that off, if you need to recalibrate the dial to less than zero.

I found that the higher the speed, the higher the error. (It was maybe 3-4 MPH at 35 MPH, but was closer to 10 MPH at 70 MPH - so I recalibrated it to the higher speed.

I slapped the speedometer back in then tested it against the GPS again and it's very close at all speeds (closer at the high end, which is good). At 35 I'm probably doing 33 MPH, but at 70, I'm actually doing 70.

No, this doesn't do anything for the error rate in the odometer. But I'm less worried about that than I am about getting speeding tickets...