Hi, I know this is an old thread, but I wanted to explain that all cars have a variance between the speedometer and the actual GPS speed, this is by design, and should be around 5% difference, but not always.
each manufacturer will have different variance, Nissans tends to vary 9%-10% .. I noticed that in the Xterra, Infinity QX4, and a couple of rentals (Sunny and Altima).
I drove also Audis and VWs, and it is similar, 9-10% variance.
Pajeros on the other hand, have considerably less variance, around 3% or so, so it will seem more accurate.
Bottom line, when you get a new car, or change tire size, do check the speed variance and adjust your driving and speed limits accordingly