99 C2 indicated speed question

Is this a known fact the both the digital and analogue (speedometer gauge) speed seems to be cir. 5 mph faster than the car is actually going? I have measured this against two independent GPS units and I consistently find the 5 mph discrepancy. Have any of you found this and can the speedometer be calibrated to reflect the actual speed? Thanks, Bob

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New tires dropped mine to only 2 mph10 difference. Put new tires on

Well, if that is true would the addition of 18" wheels solve the problem? I have 17" wheels now.

The speedometer in my 996 and in my 997 read about 4 mph faster than my GPS indicates. This is also verified by radar speed signs in my neighborhood. Interestingly, my Cayenne's speedometer read the same as my GPS unit. I remember reading an explanation for this and it had something to do with German laws. There is an acceptable margin of error for speedometers in Germany, but more error is accepted for speedometers that read slow as opposed to reading fast. I'm not sure if this is true or not, but that's what I've been told.

Ran into this early on with my (then) new 2002 Boxster. Never really looked into seeing if there was any (electronic) "adjustment" that could be done to have the speedo display a more accurate speed. I have adapted to the optimistic speedo and really give it no thought. I do not know if 18" wheels/tires would make a difference. I'm not about to spend the money and suffer the harsher ride to find out. My 2003 Turbo with 18" wheels/tires reads just 2mph optimistic. But like the Boxster I have adapted. Other cars I have owned over the years have also had a slightly optimistic speedo as the car came from the factory with the factory wheels/tires. In each case I quickly characterize the speedo and then just adjust the speed to stay within the limit. if this means driving at (depending upon the car) 70mph or 67mph indicated to actually be driving at 65mph so be it. The question as to whether the speedo can be recalibrated or somehow tweaked to reflect true speed is a question you'll have to put to a tech at the local Porsche dealer service department. The car's electronics (instrument cluster controller I guess) might support some correction factor that can be applied by using the Porsche diagnostics computer.

Yes, it's true, all speedo's read fast, and I did hear it was due to German laws. Maybe with newer cars they have worked their way around this with electronics. It has to do with new vs. old tire OD, and the fact that the speedo sensor comes off the transmission or something else in the drive train. It can't know what the OD of the tire is and must always read conservative. With new tires = max OD, with old tires = min OD, it has to be set up so it does not read high with new tires.

The analog speedo in my '00 C2 is about 5mph lower than the digital speedo. Is this also normal? Even if they arent accurate, it would be nice if they were consistent!

If your speed changes when you mount new tires, you purchased the wrong tire size. If you increase your wheel size, the tire sidewall height will have to decrease by a corresponding amount to ensure your speedo remains accurate (or, more precisely, remains intentionally inaccurate). The indicated vs. actual speed is 3MPH different for me, which is in line with what I've read. Since indicating a lower than actual speed would be potentially dangerous (and would delight attorneys worldwide), automakers display an indicated speed plus the margin of error for the measuring device. If the measuring system has a +/- 1% accuracy, the automaker will add 1% (and possibly more) to the measured speed. This ensures that even if a particular vehicle miscalculates 1% lower than actual speed, the indicated speed will still be correct. Moral of this story: if you intend to jump your vehicle over a canyon, use a GPS speedo.

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My research into this topic suggested that this cannot be set, which surprised me. I can reprogram the speedo drive ratio for my '96 Corvette with a $100 programmer, so I never imagined that I wouldn't be able to do the same on a Porsche.

I see about 2-3 MPH difference on the digital speedo, and there is about a 5 MPH difference on the analog speedo. And yes, new tires should change this to some minor extent but perhaps not noticeably.