Is gps mph measurement truly accurate?
Nick
It will be much more accurate than a car's speedometer - assuming a 'modern' GPS unit with a good fix onto multiple satellites.
Inaccuracies arise with gradient, since GPS horizontal position fixes are more accurate than their vertical altitude fix. GPS will therefore tend to understate distance and speed if driving up or down a gradient. Fag-packet Pythagoras shows that a 4% slope will cause a GPS error of <0.1% and the error only becomes significant (ie greater than 1%) when the gradient is steeper than 15%.
