[Ajack]

I copied this from a public forum:

I think you're correct to favor the GPS device's speed reading. Speedometers are often inaccurate by one to three miles an hour for a number of reasons ranging from tire size and wear to irregular calibration at the factory. The indicated speed is usually higher than the car's actual speed. Industry officials have said speedometers are set "optimistically" to give drivers an inflated sense of speed (and help them avoid tickets). GPS devices record distance over time without the effects of mechanical variables and thus tend to be more accurate.