Boxster S speedometer calibration

[Nimble70]

My 2002 Boxster S's speedometer reads fast a constant 4 mph above 30 mph. (That and my Valentine V1 probably account for my lack of speeding tickets --- yet. ) Strangely, however, the odometer is spot on when compared to my GPS. Apparently on the 2002 the speed signal is picked up from the ABS sensor on the right rear wheel. This sends a pulse train to the computer which converts it to an analog voltage sent to the speedometer. I have no idea how the odometer works if not counting revs of the wheel multiplied by the effective rolling circumference. The Porsche dealership tells me the speedo cannot be calibrated, which I have a hard time believing. Anyone else run into this problem or have an explanation why the discrepancy between the speedo and the odometer? Any way to correct for the 4 mph speed error, other than mental subtraction of course? I'm running 18" wheels with factory recommended tire sizes on both front and rear.

[mikefocke]

Well known fact of life on all Porsches. IIRC has as much to do with govt penalties for delivering a car whose speedo reads lower than "actual" speed as anything else. When GPSs became popular, this was a frequent topic on the forums.

My '01S reads the same.

[Nimble70]

Thanks, Mike. Any thoughts on why the odometer is accurate and the speedo is not, or is this something also inherent in the software? Guess I need to figure out how to hack my car!

[Wayne 962]

Originally Posted by Nimble70

Thanks, Mike. Any thoughts on why the odometer is accurate and the speedo is not, or is this something also inherent in the software? Guess I need to figure out how to hack my car!

They might just adjust down the speedo level as Mike indicated, but leave the ODO measurements alone. The odometer is a pure function of total number of clicks - the speedometer is clicks per time cycle, so if you subtract 4% from the time cycle, you get a faster speedo, but it doesn't affect the odometer.

Just guessing here though...

-Wayne

[Nimble70]

Thanks, Wayne. That explains the difference between the speedo and odo. Now I just need to find someone who knows how to make the adjustment. Any suggestions for folks adept at reprogramming Porsche ECU's and other electronics?
It drives me nuts that my Ford Explorer has a better (more accurate) speedo than my Porsche! I find it hard to accept that a car as finely engineered as the Boxster S should have this sort of annoying glitch. Go figure.

[tholyoak]

The 'error' is done on purpose due to European regulations as mentioned above. If you look at the speed reported by the wheel sensors in the DME or ABS control units it is accurate. A fudge factor is applied in the speedo itself.

[BYprodriver]

Maybe Porsche wanted to reduce owners lost because of too many speeding tickets

[michaelgalassi]

I tend to reject the theories above as the car knows EXACTLY how fast it's going. Connect an ODB-II display (eg scan-gage 2) and you'll see that it matches up very closely (within a percent) with a GPS. This is probably Porsche trying to make you feel good by adding 6% (on my 2001 Boxster S) to your speed, after all, you didn't buy a Porsche to go slow like everyone else does. I would LOVE to learn how to correct this, perhaps with the Durametric cable/software?

[thompsondrew]

I normally drive 5 mph above the speed limit. With my Boxster I drive 10 mph over the speed limit. Case closed.
I too found this out with my GPS.

[DonMunro]

Hi guys,
Thanks for the info. I just got a 2002 Boxster S... my first Porsche.

It's on 19" wheels and +1 tires so should be pretty close to factory rolling radius BUT the speedo is ~ 9% high. Very annoying

On my previous 2 cars.. PT Cruiser GT and VW Cabrio the service guys at the dealership just went out with a GPS and dialed in the speedo to "dead-on". I was going to request this at the Porsche dealer, but it appears that would be a waste of my breath

cheers
Don