I've had reason to look into this recently and I'm afraid that Oneyedvic and idalovett are mistaken.
Sat-nav systems measure your speed over the surface of the earth without taking contours etc into account. In effect, they work by using line of sight distance and time of travel between points to work out the average speed of your vehicle at a given moment.
None of us travel from A to B in this manner, because no road is perfectly flat, straight and contourless. The only accurate way to measure speed over a given distance is via a speedometer. Because of this, what Oneyedvic said in the second paragraph needs swapping around ie if your sat-nav says you're doing 80mph, you're probably doing 88mph. Your experiences with this 5-8 mph drop chris1970 sound about right.
The police will not accept a sat-nav reading as an accurate reading because of what I said earlier. They rely on the speedometer reading. Furthermore chris1970, there's been at least two test cases in the courts about this where a driver tried to claim his sat-nav showed he was travelling below the speed limit.
Expert witnesses were called on behalf of the prosecution and they demolished the case for the defence on the basis of this curvature of the earth. It's now gone down as case-law, which means anyone trying their luck with this defence in the future is in for a nasty surprise.
I'm not disputing that in the right circumstances, they can show the same speed as your speedometer. The problem is that they don't do it all the time and this innacuracy and uncertainty is enough to get booked. Besides, you haven't got a chance if your caught in a radar trap.