Dengie Boy, I haven't seen anyone respond to your question about the GPS. I'm sorry I can't help either, other than to say that my CB's speedo says I'm going faster than the GPS, but the GPS takes a while to react to any throttle variation, so I'm not sure how much you can rely on it.
At one point I had a iPhone app called "QMT" which allowed you to run a quarter mile and collected all sorts of data about the run. That might help.
However, the error you reported seemed excessive to me.
I suppose you could ask a friendly police officer to point a speed camera at you.