That's why i suggested downloading that app Bob - it's a GPS speedo so gives a benchmark.
There are technical limitations to GPS speedos though, they assume the earth is flat
so if you're going down (or even up) a steep hill, the speed could be pretty different to what the GPS reads.
Think about an Iscosceles triangle, the point of the triangle being the satellite and the two base corners being the start and finish positions - that is how the GPS speedo sees things with the car on a paper map with no contours. Factor in the slope and the speed could well be different!
