Is it common to have the odometer and trip odometer continue to function but not accurately record distance travelled? If so what is the solution? Is this a mode that eventually leads to complete failure of the odometer function?

Yesterday morning I filled up the gas tank and reset the trip odometer while stationary. I ran a few errands covering a couple miles. My wife and I took the car to a PCA event last night that according to the Allstate Drivewise app it was 29.5 miles there and 30.0 miles back. At the end of the night my trip odometer stood at about 35 miles when it should have read about twice that.

I did a test this morning and drove another car 11.6 miles. My 928 recorded 5.3 miles for the same trip.

I have had the car about 5 months. It is a 86.5 US model with the auto trans, Porken chip, x-pipe, rear muffler delete. The previous owner of three years said he had gotten 450 miles from a tank of gas. I assume that was on a road trip out west where he lives.

My fuel gauge reads low and the low fuel warning light works when the ignition is turned to the first position, but not when the tank is actually low on fuel. This is a separate problem I have to address. In conjunction with not having accurate fuel level readings I have run out of gas twice, once with 320 miles showing on the trip odometer and most recently with 215 miles showing.

Overall the car runs great, has no leaks and had been thoroughly inspected a few weeks after I took delivery by a knowledgeable mechanic here in Atlanta (Howard Wright @ European Classics).

 

based on your report the speedo gear is failing inside the pod.
And you need a new fuel sender and cap seal.

Roger has a less expensive 944 sender that does the trick,
and also sells a replacement speedo gear for the DIY guy

 

Thanks Stan.

 

I agree with Stan. I never go over 280 before refilling and on long trips maybe 300. Our 86.5's do get better mileage than the others but 400+ is tough to swallow and I a left coast driver. The #1 reason that I don't run the tank dry is that we are driving 30+ year old cars and at the bottom of your tank there is a lot of crud. The filter takes care of most of it but if you run it to where it can start sucking out dirt and whatever you are going to create more issues. Also stick with top tier gas Chevron or the like and throw a can of Tekron in every 10 tanks or so. Stan is correct on the other issues.

 

I'm not sure how the circuitry works with this, but when I bought my 82 Bamboo Beige 928 the speedo was not working. Later, when I got around to preping it to drive I checked on here and my attention was then focused on the connection for the speedo in the spare tire well. I dug down to it and found that I wasn't sure how to disconnect it but I wiggled it around some and then put it back. Then the speedo was working. Now, however the speedo has some intermittant glitches and my odo and trip miles are doing just what yours are doing. For now I think I an going to pay closer attention to the connection and try harder to get it apart and clean it and see if that might solve the problem.

 

Jerry, regarding your reference to the speedometer drive, is this shredding cable part of that system? This is above the left rear axle shaft.

 

Remove the carpet in the boot, the cover over the spare wheel, and the spare wheel itself. On the forward right corner there's a plastic cover held on by a nut over a stud welded to the tub. Remove that cover to expose the connections from the speedo sender, plus the ground point for the rest of the electrical stuff in the area like the fuel pump, the neutral safety switch on automatic cars, and the gear selector position indicator also on automatic cars. Clean the ground point. The connectors are all "self-wiping" contacts but still deserve a spritz of DeOxit after they are disconnected. The connectors and the ground point there are the Most Likely suspects for poor connections in the speedometer sensor/switch circuitry.

I don't have pictures handy, unfortunately. Hope the description is adequate.

 

FWIW...Tony Callas stated that when resetting analog trip odometer it's best to reset it when parked not when moving...he felt it prevented stripping plastic gears. I adopt this method