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NJ924 · Joined: 13 Jun 2004 Posts: 20 Location: New Jersey

The speedometer on my '88 924s reads about 10% below the car’s actual speed. I checked it by making multiple passes by a police road sign ("The speed limit is 55 mph. Your speed is...").
Any ideas what could cause such an error or if there is any way to calibrate the speedometer?

Paul · Posted: Thu Jun 02, 2005 2:24 am Post subject:

Two causes:

Non stock front tire size
Speedo error.

A speedo shop can adjust it. All they do is move the needle.
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Owned and repaired 924s since 1977
Porsche: It's not driving, it's therapy.

NJ924 · Joined: 13 Jun 2004 Posts: 20 Location: New Jersey

I think the tires are the right size: Pirelli P6000 195/65 R15. The sappedo needle rests on the stop at the bottom, so I'm not sure tif it can be adjusteed down.
I wondered if a high resistance ground would make it read off. Any other ideas? Thanks

Paul · Posted: Sat Jun 04, 2005 2:10 am Post subject:

The speedometer is a mechanical device, so a bad ground cannot effect the reading.

As you have stated, when the needle is on zero, it's against a pin. If you were to remove the pin, the needle would move much lower than 0. So it can be adjusted.

Sometimes this error is caused by a novice fixing the odometer. In order to disassemble the unit, the needle must be removed. A novice just pulls it off while it's resting on the pin, and therefore has no idea of where to reinstall it.

One needs to spin the cable at a speed that causes the odometer to record a mile a minute, then place the needle on 60.

Smoothie · Posted: Wed Jun 22, 2005 6:41 am Post subject:

I read somewhere that they're calibrated by adjusting the amount of magnetism of some part.
You should see a small white line at the outer edge of the gauge face, a bit counterclockwise past the pin. I noticed on 2 different speedos that if I lifted the needle, moved it past the pin and let it settle, it would point at that small line. Seems to me, that a shop with the correct equipment would first make sure that the needle points to that line when at rest, then lift and sit it against the other side of the pin (without removing the needle), then hook it up to their calibrator and adjust the magnetism to get the needle pointing at whatever speed they're running it at.

Over time, the level of magnetism tends to diminish, leaving you with a lower speed reading.

You could have a shop with the correct equipment do it or since you know how far off it is, you could just move the needle yourself. First lift the needles' end and move it counterclockwise past the pin. -Then tap the speedo to get the needle to settle where it wants to. Then pull the needle straight up off the speedo and re-place it pointing the equivalent distance of 10mph further clockwise.
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"..it's made in Germany. You know the Germans always make good stuff."

'82 924T, US version, dark green metallic, 5 speed Audi 016G gearbox

D Hook · Joined: 02 Nov 2002 Posts: 3158 Location: Omaha, NE

Have you tried timing it between mile markers on the highway? Using a stopwatch, holding steady at 60mph on flat road, time the 60 seconds and compare it to your location relative to the mile marker. Those side of the road speed traps are not always the most accurate.

What I usually do after working on my speedo is follow behind my wife and her Tahoe and check it that way. According to the state trooper, her speedo is VERY accurate! Now, if she would only LOOK at it once in awhile!

NJ924 · Joined: 13 Jun 2004 Posts: 20 Location: New Jersey

I have driven past one of those police radar signs "the speed limit is 55, our speed is 49". I havebeen wanting to run past a measured mile and see how I fair. I assume if the odometer correctly records the measured mile, then it is just the needle.
Thanks for the help guys.

Manning · Joined: 29 Feb 2004 Posts: 151 Location: Akron O-Hi-O

Mine used to read about 10 mph too fast. I thought perhaps it was the cable drive, but when that went and had to be replaced I saw no improvement. Then when the odometer gear failed I sent the speedo off to North Hollywood Speedometer and told them what tire size I run and had them calibrate the thing. It runs spot on now based on the old stop watch and those police speed signs.

Now aerial surveillance on the other hand...
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Michael Manning
1988 924S Undergoing weight loss program

WolfeMacleod · Joined: 07 Jul 2005 Posts: 14 Location: Seattle, WA