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

10-19-2007, 09:32 PM

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.

10-20-2007, 03:56 AM

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

Chills · 10-20-2007, 04:21 AM

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.

JCL12 · 10-20-2007, 05:59 AM

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

tombea · 10-20-2007, 06:16 AM

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

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.

Lil bastard · 05-06-2009, 01:44 PM

Quote:

Originally Posted by JCL12

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

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.