Speedometer accuracy - 986 Forum - The Community for Porsche Boxster & Cayman Owners

During my drive today, I was using a GPS. The speedometer was consistently reading 8-10% higher than the GPS. Is there a way to have the speedometer adjusted/calibrated to provide accurate readings?

These cars usually read a constant 3 to 4 mph faster than actual. The odometers are usually correct. Others have reported the dealer can reduce the error somewhat, most of us just drive 10 over.

My GPS is normally reading 2-3 MPH below my speedometer. It depends, on the car, most of the speedo's are off a little, but it shouldn't affect the odometer. If you ever upgrade rims sized or something of that sort, that could cause it be off. The dealer might be able to help you calibrate it. I just go with the flow, and avoid being the fastest on the road (unless no one is around) and the exact speed doesn't really matter.

i hear the dealer can change it because its electronically controlled. In my car I can see the real speed (in KM/hr) using the climate controll special function.

My GPS says it is off (fast) by 2 MPH in most speed ranges. Same thing for my Ford Explorer.

According to my GPS and the radar/speed signs by the side of the freeway my car's spedometer is about 4 mph over my actual speed.

Very true but for most of us that doesn't have anything to do with speedometer accuracy.

can somebody explain to me how our speedometers can read 2-4 high but the odometers will still be fine? I don't uderstand!

I would like to also know that. If that is the case there would have to be two separate circuits, one for the odometer and one for the speedometer. One way to test this would be to drive a steady 50MPH for one mile. If the odometer flips at the end of that time then it looks to me like the odometer is also off. Tom

I can't answer your question about the difference between speedo and odometer but I can share my experience. I have fairly good Garmin GPS and it reads 90kph at 97kph on the speedo, and 110kph at 118kph on the speedo. This means the speedo on my 2002 S manual reads 7 to 8% too high. This is crazy given I have 265/40's on the rear instead of what the manual states should be 265/35's. This means I have bigger tyres that matematically should mean the speedo under-indicate by about 4% yet it seems to over-indicate by 7 to 8%. Must be how Porsche set good fuel economy figures and longevity data.

Mine's a 2001 S, and its speedometer is off by a constant 3-4 mph at all speeds above about 20mph. The odometer is accurate. Odometers have to be accurate as a too-optimistic odometer will run out the warranty more quickly. One of the Japanese automakers (Toyota, I think) lost a class-action suit over this odometer issue.

I used my Garmin GPS this past weekend to check the accuracy of the odometer too. It's off by a little. For a 300 mile trip on the odometer, the GPS read about 297.5 miles.

My speedo is 4MPH higher than GPS indicated speed consistently. I've also found this to roughly the same error in the odometer, when comparing trip figures from the cluster to trip legs from my GPS. Patrick

I thought it was a joke when I read that your HVAC controls could be used as a diagnostic tool, and to tell you your true speed. But, I tried it on my '98 and was truly amazed to find it was true. I checked the speed at 60, 70 and 80 mph indicated on the speedometer and sure enough, when I converted the kph shown on the HVAC I was always exactly 10% slower than the speedometer indicated. Can't say as I appreciate Porsche lying to us about the speed of our cars, but it's good to know. Thanks, guys.

Exactly how do you do this? I would like to check. BC.

http://www.cb-racing.com/boxster_AC.html

With a computer controlled system, it's easy... count actual wheel RPMs, do the calculation, and the ODO is correct. Add the 'fudge' factor (safety margin in Porsche parlance) before sending the signal to the Speedo. It's not at all mechanical, even the analog needle is actually electro-mechanical.