Speedometer vs GPS Speed

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01 Aug 2012 12:14 #9929 by RGJ
Speedometer vs GPS Speed was created by RGJ
Hi Everyone,

just got a 1988 928 S4 and noticed the speed on the odometer differs from the spped indicated on my GPS.

Any thoughts as to why?

Although it has after market rims, the tire sizes front/back are correct.

Thx

RJ

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01 Aug 2012 13:02 #9931 by denhamfly
Replied by denhamfly on topic Speedometer vs GPS Speed
:eek: I am a big boater the GPS speed should be right it is speed over ground and if you stay the same speed the delay should not matter. I would think the car mechanical one would be wrong. There should not be a big difference. The is a small delay on the GPS so if you are trying to measure ¼ mile or 0 to 60. The GPS will be incorrect. Steady speeds go with the GPS.

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01 Aug 2012 13:16 #9932 by RGJ
Replied by RGJ on topic Speedometer vs GPS Speed
thx Denhamfly,

hmm..driving is at normal driving/ highway speeds. The difference is approx 15kms.

Can this be easily fixed?

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01 Aug 2012 15:20 #9933 by denhamfly
Replied by denhamfly on topic Speedometer vs GPS Speed
I would not know why it is off, tires would be my first guess, but you stated that they are the correct size. I will bet you that some of our club members will be able to help you out. That is the number 1 benefit of this club, someone always has the right answer.

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01 Aug 2012 15:23 #9934 by denhamfly
Replied by denhamfly on topic Speedometer vs GPS Speed

thx Denhamfly,


hmm..driving is at normal driving/ highway speeds. The difference is approx 15kms.

Can this be easily fixed?


You can always time it from mile marker to mile maker to double check.

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02 Aug 2012 08:19 #9938 by Kiln_Red
Replied by Kiln_Red on topic Speedometer vs GPS Speed
The 928 speedometer almost always reads inaccurately these days. Just one of the many small 928 nuisances. Cleaning your grounds should restore the accuracy of all your gauges though. Always worked for me.

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02 Aug 2012 08:48 #9940 by RGJ
Replied by RGJ on topic Speedometer vs GPS Speed
Thx KilnRed,

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

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02 Aug 2012 08:54 #9941 by Kiln_Red
Replied by Kiln_Red on topic Speedometer vs GPS Speed
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.

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13 Aug 2012 21:34 #10089 by mm928
Replied by mm928 on topic Speedometer vs GPS Speed
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.

1983 928S - 16v 4.7L L-Jetronic
Dark Grey w/ Blue Leather
85/86 exhaust manifolds
Y-Pipe
SS Hi-perf Cat
S4 springs
Bilstein shocks
Air pump delete
VIN: WP0JB0926DS861180
- - Isn't Life great? - -

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15 Aug 2012 12:04 #10161 by wallyp
Replied by wallyp on topic Speedometer vs GPS Speed
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

Wally Plumley
928 Specialists
www.928gt.com

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16 Aug 2012 08:54 #10203 by RGJ
Replied by RGJ on topic Speedometer vs GPS Speed
Hi Wally,

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

The odometer readings are accurate.

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16 Aug 2012 09:06 #10205 by wallyp
Replied by wallyp on topic Speedometer vs GPS Speed
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.

Wally Plumley
928 Specialists
www.928gt.com

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