Just today while I was out driving I noticed the odometer not turning over, the speedometer was functioning properly. What gives?
1984 (930)

 

If these instruments are anything like the BMW units, a drive gear is broken. This seems to happen when it gets older and brittle, and you reset the mileage when the car is in motion. Can be repaired or get a used speedo...you WILL find one with the SAME MILEAGE...won't you. as a matter of fact I think I have 2 in my warehouse, but they may be metric.

 

You can replace the drive gear. Rennlist sponsor Rennbay has them on sale, see the admin posting at the top of the 911 forum. Or you can send it out to someone like North Hollywood Speedometer, who can fix it for you. North Hollywood put a 150 mph face, replaced the odometer gear, and recalibrated an SC speedometer for me, and did very good work.

 

I fyou don't want to take your speedo apart, you can sent it to either North Hollywood Speedometer, or Palo Alto Speedometer for qa rebuild. They can also re-calibrate if you are running non-OE sized tires. Usual disclaimers.

 

911 & Porsche world magazine had an article on this rebuild in the latest issue - might be worth a look, but the process did seem a little delicate.