Just a thought. I have a satnav and it always says im going 5mph slower than the speedometer says. I also know that the speedometer says im going about 2mph slower than i really am so the satnav is about 7mph out. I know that in theory it should be incredibly accurate but it never seems to be. Might just be my satnav though.

Are you allowed to use GPS to calculate speed? What about if you lose the signal or something else goes wrong?

My personal choice would be checking how fast the axle is rotating although i wonder if its possible to do something similar to a laser mouse.