Van...GPS speed is the most probably the most accuracte measurement over ground you can get. I took my hand held this winter on several trips and beyond telling me when to turn it gave me an a speed that was very close to my my speedometer. I had the truck in crusise control @ 75mph and it was saying the same on the gps.
Where you reading knots?
Speedometers read the rotation rate of a tire. If the tire is non-stock there is your problem. A small increase or decrease in diameter will result in a measurable difference in rotation rate. Even 1 inch will make a measurable difference.
I think you can get your speedometer tuned to match your tires but I am not sure... never did that. The last time I had non-stock tires it was on a 1973 Camero 454 that I sunk in a TON of money to....I knew the speedometer was off but I was 18 and look much at that guage....Boats are more fun to waste money on.