Speedometer vs GPS Speed

Thx KilnRed,

hopefully it's just the speedometer and battery, oil and fuel guage are not impacted

The coolant temp gauge, tach, fuel gauge, and oil pressure gauge are all very reliable.

I've never seen a 928 voltage gauge work right for any long duration. The speedo is hit/miss.

Mine is like almost any other car. Differs from the GPS anywhere from 2-2.75 mph. The car says I am going faster than the GPS. This is with original wheels and correct tire size. The same thing happens with by new Fiat 500 and 2 year old VW Tiguan. Although a little closer.

I suspect slight differences between tires sizes and rims for the most part. I think I have had one car that was just about dead on with a GPS. The good thing is when your speedo says you are going faster than your..especially in a Porsche. My Tiguan says I am going SLOWER than the GPS....a recipe for a ticket if not careful. 15km is a lot to be off, so I agree to make sure the grounds and contacts are clean. Use an eraser if nothing else on the contacts. Works great. The grounds need regular maintenance anyway. These gauges are pretty sensitive for the most part. So when you have good flow, they can return to good accuracy.

You didn't say whether the speedometer is reading faster or slower...

When the speeometer reads 100, what does the GPS read?
When you drive 10 km, how many km does the odo read?

Wally

Hi Wally,

when the speedo on the 928 says 100km/h, the GPS says 85km/h.

The odometer readings are accurate.

OK, the accurate odo reading eliminates one (fairly rare) problem. On the earlier cars, the speedometer and odo are driven by pulses from a set of magnets in an aluminum ring attached to the ring gear. If you lose a magnet, you lose part of the pulses, and both speedometer and odo will read low.

In your case, since the odo is correct, the problem has to be the speedometer head. You can remove the speedometer and send it to a speedometer shop (old memory - Hollywood Speedometer?) for recalibration, lubrication, etc. Or you can buy a used speedometer and hope that it works, and accept the mileage on the used unit. No real idea of cost, but I don't think that it is unreasonable.