986 Forum - The Community for Porsche Boxster & Cayman Owners - View Single Post

lagrasta · 09-30-2009, 02:57 PM

Here's the thing that really threw me off. I have a ScanGaugeII OBDII scanner that has a gauge mode that can, among other things, display the speed according to the car's computer.

I compared that, the speedo, and a gps unit and got this:
Speedo = 75 mph
Gps = 68 mph
OBDII = 69 mph

That confirms for me that the on board computer knows the correct speed and is intentionally under-reporting it on the speedo. At least, I know that the computer is reading the real road speed correctly.

I know manufacturers calibrate speedos conservatively, but I thought it was interesting to see the difference between the speedo and the OBDII readings.

09-30-2009, 02:57 PM	#22
lagrasta Registered User Join Date: Jul 2009 Location: San Diego Posts: 2	Here's the thing that really threw me off. I have a ScanGaugeII OBDII scanner that has a gauge mode that can, among other things, display the speed according to the car's computer. I compared that, the speedo, and a gps unit and got this: Speedo = 75 mph Gps = 68 mph OBDII = 69 mph That confirms for me that the on board computer knows the correct speed and is intentionally under-reporting it on the speedo. At least, I know that the computer is reading the real road speed correctly. I know manufacturers calibrate speedos conservatively, but I thought it was interesting to see the difference between the speedo and the OBDII readings.