Police will not accept GPS speed readings to contest speeding fines
GPS devices were not a certified method of tracking speed
Luddites. I'd suspect half a dozen satellites triangulating my speeding car might be just as accurate, if not much better, as their standalone revenue gathering tools.
And I suspect that Mr Plod's revenue gatherers would readily shop you in a court of law with similar 'uncalibrated' electronic data taken from your vehicle's engine management / navigation systems if there was a chance of prosecution.