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took the GPS for a ride today and was surprised to find a difference of 6mph between that two at cruising speeds (986 is too high). Any thoughts on what may need adjusted?

stock spec tire diameter

and before anyone starts questioning the accuracy of my handheld GPS it's dead on with our other three cars, saab, saab and vw. no more then 1mph difference at cruising speeds.

comments?

Shawn

1997 986

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You'll notice that many people in the forum have this same issue. Mine is right at 10% off. Also stock wheels. It's annoying but I am out of warranty and don't feel like spending big bucks to get it fixed. I just use the 10% subtraction method.

The only real issue is that I can't convince my 17 year old that I'm not speeding by that much when she sees the speedometer, especially since I tell her she better not speed at all. "Dad, you're doing 80 in a 65!"

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Another thing, i don't have a GPS but think that my odometer is correct. Did you check to see how accurate your odometer is? I would be very interested to hear your findings if you test it out.

I could have check that but cleared out the odd reading last night. I'll recheck this weekend and update.

I should also note for this post the digital and analog gauge are in sync well between the two of them.

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You'll notice that many people in the forum have this same issue. Mine is right at 10% off. Also stock wheels. It's annoying but I am out of warranty and don't feel like spending big bucks to get it fixed. I just use the 10% subtraction method.

The only real issue is that I can't convince my 17 year old that I'm not speeding by that much when she sees the speedometer, especially since I tell her she better not speed at all. "Dad, you're doing 80 in a 65!"

I have no problem living with it but I was curious if it's a simple tech2 adjustment or an actual gearing problem in the speedo assembly. Any ideas? It was rather odd it was exact over a wide spectrum of speeds I cruised at as opposed to a percentage of a given speed that is often seen with tire mismatches.

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5 MPH off seems to be common. I have seen MANY comments to that effect before.

To be honest, between the 5mph cushion and the K40, I don't worry about it TOO much....

Edited by Andy_M
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Other than GPS, are there any other methods to determine speedo error? This might explain why I constantly get passed when I feel I'm doing the limit +.

They all have the 5mph error, the speedo always reads faster- part of the package to enhance perceived performance. 10% error is lawfull - which also means the odometer is wrong by the same amount

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The only real issue is that I can't convince my 17 year old that I'm not speeding by that much when she sees the speedometer, especially since I tell her she better not speed at all. "Dad, you're doing 80 in a 65!"

That would drive me crazy... Speed doesn't kill, stupidity kills. Send her on some proper advanced driver training (not the lame "defensive driving" schools - but proper driving classes which talk through the physics, road conditions, visibility, and safe handling at high speeds)... It would help in both low speed and high speed situations, and help recognize dangerous conditions (especially when wet or icy). Then you won't have to worry about her speeding - she should know what a safe speed is at all times :)

Anyway, about the speedo... Mine is way off too. When I measure against my accelerometer and against RADAR speed sensing it seems to be around 10% off.

Thanks!

Shash

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I thought that every manufacturer makes that 10% higher as part of safety thingies.

So far I never have driven car which would show the exact speed, apart our company truck and that’s 'cos they must be recalibrated on regular bases.

Yes you can have it calibrated at any rolling road station (at least in the UK), but for me it's a waste of money ;)

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I've checked my speedo at a number of different speeds and it appears to by a 4mph offset and not a percentage of the speed.

The state of Washington has installed a number of speedometer calibration checkpoints just for this purpose, they look like a speed limit sign with a digital display 'Your Speed IS"... (-: They seem to work at almost any speed.

As I understand the 986 system it is not adjustable and can't be recalibrated. If some one online knows how to adjust it, please post as it looks like some of us would like to work on it.

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Mine reads 118kph for a real speed of 109kph measured with radar gun.

I don't believe it's a margin of safety thing. It's not necessary. It doesn't work coz people work it out and make up for the difference anyway. It only introduces more error and takes concentration off the driving to correct for the difference mentally. Besides, no other manufacturer does this. My other cars are off by about 2kph at most at 100kph.

I don't believe it's random error either. No one has ever reported the speedo reading 10% under the real speed.

There is no reason to believe the odometer is correct if the speedo isn't, unless Porsche delibrately made the speedo read faster.

Why would they do that? My answers don't defend Porsche the least bit.

How can you make it read accurately? I think increase the diametre of your wheel/tyre combo by 10% ought to do it.

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I was talking to a Volkswagen engineer at the Belgian assembly plant in Vorst (recently converted to Audi) and the guy told me that they calibrate the speedometer to show 5% more than the actual speed. Just to handle future changes, there is tollerance on about everything and this would make sure the actual speed is not higher than shown.

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I was talking to a Volkswagen engineer at the Belgian assembly plant in Vorst (recently converted to Audi) and the guy told me that they calibrate the speedometer to show 5% more than the actual speed. Just to handle future changes, there is tollerance on about everything and this would make sure the actual speed is not higher than shown.

Future changes, changes of what?

That is an unexceptionable discrepancy here in the states. I can see no reason to be that far off. I've driving 1001 cars and have never seen another make/model consistently this far off across the model line!

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I was talking to a Volkswagen engineer at the Belgian assembly plant in Vorst (recently converted to Audi) and the guy told me that they calibrate the speedometer to show 5% more than the actual speed. Just to handle future changes, there is tollerance on about everything and this would make sure the actual speed is not higher than shown.

Future changes, changes of what?

That is an unexceptionable discrepancy here in the states. I can see no reason to be that far off. I've driving 1001 cars and have never seen another make/model consistently this far off across the model line!

*flame coat on*

That can happen only in America

*flame coat off*

:P

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I was talking to a Volkswagen engineer at the Belgian assembly plant in Vorst (recently converted to Audi) and the guy told me that they calibrate the speedometer to show 5% more than the actual speed. Just to handle future changes, there is tollerance on about everything and this would make sure the actual speed is not higher than shown.

Future changes, changes of what?

That is an unexceptionable discrepancy here in the states. I can see no reason to be that far off. I've driving 1001 cars and have never seen another make/model consistently this far off across the model line!

*flame coat on*

That can happen only in America

*flame coat off*

:P

You lost me with your poor attempt at humor...

Speeding and driving laws are so tight in some areas of the USA that 5mph will net you a $150 fine. I doubt the judge will care that the Boxsters are made with a 5% tolerance as you try to explain you were doing compensation in your head because the Germans have some crazy theory why 5% off is better then greater accuracy. Is it really that hard to get a speedo to match real world speed and what need is there for a 5% error factor.

Edited by rsfeller
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