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Speedometer Error

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The city where I live has recently installed radar speed displays all around town. The speed on the displays match exactly with the speedo in my Mercedes.

However, the speedometer in my 2001 996 Turbo is off by about 10%. When the speedo shows 33 mph, the radar shows 30, when it shows 44 the radar shows 44.

I have the proper size tires on the car so I have dismissed this as the source of the error.

Is there any way to adjust the speedometer in my Porsche. I ask because I know I can (somewhat) easily adjust the speedometer in my Mercedes.


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Thank you very much for the answer.

So Porsche is protecting me from myself, wow. Is this common to all Porsches, just Porsches built after a certain date, or just certain models?

So is this a 4 mph error as indicated in the Pelican forum or is it 4%?

BTW I searched this forum for anything that contained "speedometer" with no luck before I posted my question. I hate to reinvent the wheel.

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As has been said, it is typical for the speedometer to over-read by the amount you are experiencing.

I don't know how many people have thought of this, but I think this phenomenon impacts on the odometer reading. I think that the actual vehicle mileage could be 5-10% lower than the odometer reading. The DME on my 996T also recorded the vehicle distance travelled in km, and this was less than the odometer reading.

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