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

admlmm · 10-22-2007, 04:37 AM

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.

indydave · 10-23-2007, 05:44 AM

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.

10-23-2007, 06:36 AM

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.

Cloudsurfer · 10-23-2007, 07:35 AM

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

schnellman · 05-06-2009, 09:10 AM

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.

Bladecutter · 05-06-2009, 10:07 AM

Quote:

Originally Posted by schnellman

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.

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

BC.

rick3000 · 05-06-2009, 10:34 AM

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