Most trafficlaw enforcement agents use radar and not satnav readings to determine the speed of the subject. So it is not advisable to use the satnav as the means of determining one's speed. Furthermore, satnavs have an inbuilt error, as they do not resolve down to a fine detail; only to a few metres. One can see this if one deviates from the course chosen by the satnav. It takes several metres of deviation until the satnav snaps back onto the actual road upon which the car is travelling. The error is typically 3 metres.
Another error with satnav is the vertical distance. Travelling up or down hill will introduce further errors in the satnav speed reading.
Speedometers are designed to never read less than the actual speed, and are permitted to read up to 110% of the actual speed + or - 6mph.
Edited by: crossbow