Odometer & Speedo about 10-12 percent high

[Maxhr203]

My spedo is always 10-12 percent high. This is also true for the odometer. My 1999 996 has 18,459 miles, but as much as 2,215 are overstated. What can I do to fix this for the future? It is a pain to always figure 10% and guess at my speed.

Thanks for any help!

[eastriver]

interesting problem.

so in theory, one can change to a "slower" speed sensor so the milage build up slowly. :)

[pchak]

Funny, my '97 Boxster ran 4-5 mph higher. I thought this problem went away in '00.

[Maxhr203]

interesting problem.

so in theory, one can change to a "slower" speed sensor so the milage build up slowly. :)

60 highway miles will put just at 67 miles on the odometer!

[nick49]

About 3 years ago I checked the speedo in my wife's Boxster against my GPS and found that it was about 10% higher than actual speed. The dealer corrected it and now it is very close.

[nycebo]

I think it's a pretty well known fact that speedos typically overstate speed. However, it hadn't even occured to me that the odometer would be affected as well. I'll have this looked at in detail when next in service. I'm not saying I have a problem, but why risk it?

[0586slb]

Are you sure that the odometer is overstated? Have you checked the odometer over an accurately known distance or against GPS? I ask, because it is known that European Manufacturers factor in a higher reading for the speedometer - they're loosly allowed up to 10% over-read, with 0% under-read. However, with the over-read, the odometer is supposed to be accurate as far as they can control wheel and tyre diameters.

[jmk12345]

Do Porsche change the factory setting depending on which wheel and tyre combo they ship the car with? I have a 2002 Targa (UK) on the 18" sport design wheels but I guess the 996 was developed (and calibrated?) with 17" wheels and probably with a lower rolling radius.

My speedo is just under 10% over reading at 100mph and over 10% at 30 mph against GPS. However my current GPS reads differently to my previous GPS (on the same car) so I guess some GPS chips may be out also - anyone else notivced this?

[0586slb]

Do Porsche change the factory setting depending on which wheel and tyre combo they ship the car with?

Generally no, because usually when you increase a wheel rim measurement, you use the 'Plus one concept' - without going into the mathematics of it, if a chassis was designed with 17" wheels, when the rims are increased to 18" you have a corresponding reduction in aspect ratio (profile) of the tyres, so the rolloing circumference remains roughly the same.

[aracerx]

Do Porsche change the factory setting depending on which wheel and tyre combo they ship the car with?

Generally no, because usually when you increase a wheel rim measurement, you use the 'Plus one concept' - without going into the mathematics of it, if a chassis was designed with 17" wheels, when the rims are increased to 18" you have a corresponding reduction in aspect ratio (profile) of the tyres, so the rolloing circumference remains roughly the same.

The "10%" factor in speedo "error" has been an "achilles heel" for Porsche cars for most of it's existance! I guess, we should all assume ALL 911's (in particular) have app. 10% less mileage than appears in front of them! :) Mark

When one of my clients tells me of achieving 153 MPH on the back straight of Watkins Glen, I assume it was 143 :)

[trauh]

I took a roadtrip last week and used a Garmin Navigation system. It included a dashboard that hows your speed using the GOS satellites (this unit uses the newer WAAS technology). When my dashboard showed a speed of 85 mph, the nav unit showed it as 83-84. So assuming that accurate speed means accurate odometer, I guess I am in good shape.

Do Porsche change the factory setting depending on which wheel and tyre combo they ship the car with?

Generally no, because usually when you increase a wheel rim measurement, you use the 'Plus one concept' - without going into the mathematics of it, if a chassis was designed with 17" wheels, when the rims are increased to 18" you have a corresponding reduction in aspect ratio (profile) of the tyres, so the rolloing circumference remains roughly the same.

The "10%" factor in speedo "error" has been an "achilles heel" for Porsche cars for most of it's existance! I guess, we should all assume ALL 911's (in particular) have app. 10% less mileage than appears in front of them! :) Mark

When one of my clients tells me of achieving 153 MPH on the back straight of Watkins Glen, I assume it was 143 :)

[WetherbyPorsche]

I was stopped by the police in France 3 weeks ago.

My speedo was set-up on the cruise control at 166km.ph (103.2mph) while the laser gun of the police officer did say 159km.ph (98.79 mph). This is 4.4% lower than the mesured speed, and in the 10% range.

JC

[yeeha68]

Took a roadtrip today (98 boxster) and noticed the same thing. Using a Garmin GPS I was generally showing a 5 MPH difference. Gauge showed 85MPH, GPS read 80MPH. Now, I am a true believer in German precision, BUT, I have to say, I know the GPS was correct and the car was off.

Edited August 31, 2006 by yeeha68

[fasterjames]

My 02 is the same way. For the first few weeks I drove it I felt like I was crawling along. I figured it was just because I was more nervous about getting pulled over. Then I passed a stationary radar gun and it said I was going 38 when my speedometer stated I was going 42. In my mind I figured that perhaps the radar wasn't calibrated until I got in this great forum. Now I am certain it is off and fear that perhaps my odometer is off too. I am going to try testing it against GPS and modify the speedo if possible.

[1999Porsche911]

I have tested several 996's and have yet to find the odometer off by very much at all. The speedo can be off as much as 10%, but the odometer, at least on my Porsches and others I tested, appear to be close to miles driven. IF your odometer is substantialy off running stock tire size, prove that to Porsche and they will extend your warranty mileage. (at least in the States) Odometer uses the unadjusted signal as determined by the computer.

Edited September 11, 2006 by 1999Porsche911

[Jack Bego]

My 02 is the same way. For the first few weeks I drove it I felt like I was crawling along. I figured it was just because I was more nervous about getting pulled over. Then I passed a stationary radar gun and it said I was going 38 when my speedometer stated I was going 42. In my mind I figured that perhaps the radar wasn't calibrated until I got in this great forum. Now I am certain it is off and fear that perhaps my odometer is off too. I am going to try testing it against GPS and modify the speedo if possible.

I participated in a rally over the past weekend and found my speed and distance traveled to be 7% higher than the "official" rally data for the route. This was confirmed using my Garmin 2610 yesterday.

[1999Porsche911]

My 02 is the same way. For the first few weeks I drove it I felt like I was crawling along. I figured it was just because I was more nervous about getting pulled over. Then I passed a stationary radar gun and it said I was going 38 when my speedometer stated I was going 42. In my mind I figured that perhaps the radar wasn't calibrated until I got in this great forum. Now I am certain it is off and fear that perhaps my odometer is off too. I am going to try testing it against GPS and modify the speedo if possible.

I participated in a rally over the past weekend and found my speed and distance traveled to be 7% higher than the "official" rally data for the route. This was confirmed using my Garmin 2610 yesterday.

If that is true, then simply get the odometer (on stock rims and wheel sizes) certified by a licensed company to be off XX%. Provide this to Porsche in a certified letter as well as provide it to the dealer and have it noted in their files and chances are, you will be covered under warranty for the percentage of mileage over the difference.

I still find it hard to believe that running stock wheels and tires sizes show much diffeence between odometer reading and actual miles drivien. I have yet to test a car that shows more than a 1% difference. Again, the incorrect reading of the speedometer is UNRELATED to the calculated input used for the odometer.

[auto-xr]

My 02 is the same way. For the first few weeks I drove it I felt like I was crawling along. I figured it was just because I was more nervous about getting pulled over. Then I passed a stationary radar gun and it said I was going 38 when my speedometer stated I was going 42. In my mind I figured that perhaps the radar wasn't calibrated until I got in this great forum. Now I am certain it is off and fear that perhaps my odometer is off too. I am going to try testing it against GPS and modify the speedo if possible.

I participated in a rally over the past weekend and found my speed and distance traveled to be 7% higher than the "official" rally data for the route. This was confirmed using my Garmin 2610 yesterday.

If that is true, then simply get the odometer (on stock rims and wheel sizes) certified by a licensed company to be off XX%. Provide this to Porsche in a certified letter as well as provide it to the dealer and have it noted in their files and chances are, you will be covered under warranty for the percentage of mileage over the difference.

I still find it hard to believe that running stock wheels and tires sizes show much diffeence between odometer reading and actual miles drivien. I have yet to test a car that shows more than a 1% difference. Again, the incorrect reading of the speedometer is UNRELATED to the calculated input used for the odometer.

I did a comparison of the 225/265 vs. 235/285 setups. There's a difference but not all that much.

Edited January 23, 2007 by Loren
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[Izzy]

If any of you guys owns a late model Honday Odyssey, you probably got a letter from a certain court telling you about the class action suit Honda just agreed to settler, based on the fact that the odometer are 4% off. The settlement entitles you to extended mileage warranty in your car. I got one (2006 minivan) and keep it in glove box for future, as dealers will honor the settlement.

Not sure if Porsche has the same issue or is it speed only?

Let me know if you want more detail, I can get to the letter when I get home tonight. I don't drive the van..... me wanna go fast.....