I've just borrowed a friends Tom Tom for Le Mans and it's the first time I've had Sat Nav in the car and have an observation.

I know std speedo's are crap, but I expected the BMW's to be vaguely accurate. According to the Tom Tom the speedo is under reading by a good 7mph and this is at speeds around 60-80.

So how accurate are Sat Nav speed read outs and how do they compare to proper timing gear and speed camera's.

I understood that Datron etc also used GPS. Does the Datron stuff have quicker sampling rates for more accuracy or is the Tom Tom reading pretty close and the BMW's speedo is actually crap?

Whats your experiences? Going by the Tom Tom I could be driving faster and not worry about it

Ta