GPS speed jumps around quite a bit--I always assumed that this was because your absolute position is only known to within 10 meters, and the error varies. If your device grabs two measure that vary toward each other, voila, you just got a burst of speed. And if it captures that as your Max, there you go.
As T37 said, the average would be the better number.
2000 Beneteau 331
Northern Chesapeake Bay
Ambition is a poor excuse for not having enough sense to be lazy ~ Steven Wright