I took my portable GPS on a trip and noticed that my speedometer is 5 mph too fast. I had this problem with a 996. Is this an easy fix? These are the tires that acme with it.

 

"They all do that sir" It's one of those things they do to give you a slight cushion against the law and also because it would be illegal to under read the speed.

 

Had this problem with my 99 996, which was leased. Turns out my speedometer and odometer were off..about 5 mph on the speedo. Tested the odometer on several measured miles and it was off about 1/10th per mile! These were the OEM tires and wheels. When I did the math, it turned out my mileage was about 3600-4200 too high, and on a 36 month, 12,000 mile per year lease, this meant I was screwed out of about 3-4 months of driving. When I contacted PorscheNA, they sent me an official letter telling me a 5% variance was within tolerances. As this was the end of my lease, there was nothing I could do.
I would take it to a dealer pronto to get it checked, documented on their computer, and perhaps recalibrated, keep your reciepts etc. Make sure you do your speedo/odo homework on the road before you go. Easy to do on a measured mile with a stopwatch and cruise control. As far as I'm concerned, the speedo differential is almost incidental to the inaccuracy of the odometer! The odometer inaccuracy is unacceptable on a $100k car. Good luck. Stick to your guns!

 

Putting bigger tires on the rears will help.

 

The odometer should not be off, just the speedo.

 

Has been discussed herein before. I have a 2.8 mph optimistic readout.

 

Why? Aren't they using the same input from the drive train?

 

Yeah, but you think with a digital speedometer, in a Porsche, that it would read what speed you are actually going. My 996 was 12mph off and they corrected it to the real speed. It was not much trouble to do that if I recall.

 

It's just counting tire revs. If the tire circumference is not "nominal", due to wear, pressure, wrong size, etc., it'll show the wrong speed/mileage. GIGO...

One of these days, they'll go to a GPS-based odo/speedo and you won't have these problems.

 

No they don't. The climate control screen speed is yours actual speed and correlates to the odometer.

 

Hi,
Here is my link on the subject.

https://rennlist.com/forums/997-forum/406456-75mph-equals-72-1gps.html
Paul

 

Thanks Paul, It would be the next step to have the speedometer hooked up to the GPS, if the car had GPS. These GPS's that can get multi satellites are quite accurate. But I imagine that there would be situations where that wouldn't work. Maybe a system where the the seedo is updated by the GPS would work but alas it would be a 7 grand option that no one would get.

Porsche is about precision so having the speedo off by a couple of % when there is a digital readout seems to be not holding to the true standards of the company.

 

Oh, I don't know... seems like the gauge that matters (you know, the one in the center) is probably accurate.

If you have a choice in designing the speedo, wouldn't you (or your lawyers) push the bias toward reading high? Given all the variables in speed measurement, I'd expect some bias and build that in so that the speedo rarely (if ever) reads too low. As a result, it'll almost always read too high.

 

Then we should correct the posters who say that they went a 150mph today by saying "well not really"


But your right p-cardriver, it's better to error on the high side.

 

On the subject of speedometers, I noticed that the speedo also shows the speed in reverse. Is this unique to Porsches? I never noticed it on other vehicles.