You probably went under a tunnel or into some trees Rob...the unit will have lost some satellites and got innacurate, then when it picked them up again it worked thought you'd jumped to the new place from the old if you see what I mean....
I've had 450+mph on mine, and I'm sure there's a piccy of a SPIII with over 1200mph on it from a bike ride
Oops, sorry, what I meant to say was wow, you da man....that was quick then!!!