Odometer off on an '82 911 with a '78 Engine and Tranny

[redstormcoach]

I noticed that the odometer is running about 7mph faster than I am actually going. My mechanic told me that due to the difference in the gearing between a '78 and and '82 the odometer will be off. He suggested a fix either sending my odo out to get recalibrated to match the '78, buying a new one and having the mileage set to the mileage of the exisiting odo or buying a used '78 odo and getting that matched to my exisiting odo. What do my fellow Pelicans believe is the best fix or solution

[redstormcoach]

bump. Anyone?

[javadog]

Well, how about calling someplace like North Hollywood Speedometer and asking them the prices for doing the things you list? Then you can peruse the used parts board to get an idea of used speedo prices and figure out which alternative is cheapest for you. You might also consider whether you want a different speedometer, as there are several that will work and they have different scales and some are easier to read than others. Personally, for an SC, I like a plain 150mph one with no metric markings. Another thing to consider is how nice yours is. Does it need cosmetic refurbishment?

There's no single right answer.

JR

[the]

Assuming you are talking stock, US cars, there's no difference in gearing between a 78 and an 82.

[john walker's workshop]

the speedo sensor on the trans reads a magnet filled disc that turns with the ring gear at wheel speed. unless you changed your tire size, the speedo would not read differently.

[javadog]

I think what he meant was that his speedometer reading was off, not his odometer, as stated. That's pretty common, in my experience. None of my cars are accurate at highway speeds.

JR

[redstormcoach]

Yes. I meant my speedometer is off. How can I correct it? Being that is actually reading faster than I am going, does that mean the odometer is registering more miles than I actually have?

[ischmitz]

The odometer is calibrated to the 8 pulses per wheel rotation. Unless you changed wheel size it will be accurate as John said.

The speedometer is essentially a frequency to current converter. The needle measured a current and deflects accordingly. The electronics is based around an amplifier and some discrete components that can age with time. Especially the capacitors used can dry out. Some Speedometers have a little access hole in the back behind which you find a trim pot. That trim pot can be used to adjust the scale sensitivity.

If your Speedometer has that trim pot you are lucky. Get a second person and a handheld GPS. While one does the driving the other can do the adjustment with the Speedo out. Hope that helps.

Ingo

[Grady Clay]

Sounds like a good excuse to take the 15 to Lost Wages. A friend with a stopwatch and the highway mile markers are also good for determining speedometer accuracy. Repeat the measurement many times and do some statistical analysis. From that data N Hollywood can adjust your instrument exactly.

Best,
Grady