[helpbygrace]

Is this speed measured by GPS or vehicle?

In my experience, even though speedmeter in the instrument panel shows 10mph over, actual GPS speed is around speed limit.

[jimmcslim]

According to the Wikipedia page on speedometers, under the Error heading...

'Vehicle manufacturers usually calibrate speedometers to read high by an amount equal to the average error, to ensure that their speedometers never indicate a lower speed than the actual speed of the vehicle, to ensure they are not liable for drivers violating speed limits.', although no citation is currently provided.

Error is introduced by tire diameter being different than what was assumed for initial calibration... i.e. wear on the tires or tires that are under/over-inflated.

http://en.wikipedia.org/wiki/Speedometer

[Someone1234]

Civilian GPS alone isn't accurate enough to be believed to within 10 MpH. Even with the fuzzing gone:

http://www.maps-gps-info.com/gps-accuracy.html http://www.gps.gov/systems/gps/performance/accuracy/

[ewoodrich]

They really shouldn't differ by that much, your car likely has a speedometer that is out of calibration.

[ianstallings]

But how do you know the actual speed you're going to gauge the accuracy? Have you timed this over a set distance or something?