Originally Posted by Crowz

Actually no. The first reason I installed nav in the first vehicle I did it to was to record gps mph readings. The logs it produces are admissible in court and will trump a radar gun. I have used them before to beat speeding tickets. That's one handy thing with the android units and most other newer nav systems. I go by the gps's speed reading always and just compensate on the speedometer.

Basically I look at the gps software's reading and then see how far off the speedometer is and then go with it from there.