Until we moved to Switzerland a few years ago, I used to time speed events for the RAC MSA and ACU, whilst the GPS is an accurate system, a deviation of 1 or 2 mph against a speedo reading is quite normal, the accuracy of out timing equipment (Tag Heuer) was internationally recognised and calibrated and our measured distances were accurate to 6mm over a mile, an accuracy I don't think can be achieved with a GPS. Also the wear on tyres, even tyre sizes can make a difference to speedo readings, so if you are within a couple of mph I would be happy with that.
