walight01 said:are we talking about normal Gps where it tells you where to go and such, if so, thats not a good arguement either, most b/c they are accurate up to 5m at best, so i would say the ONLY way to truley know is to have some clock you with a good radar gun.
a gps is VERY accurate, it uses satelites to get your speed. and yes they tell you where to go and such.