Interesting discussion on the accuracy of GPS devices.
For info when I did my speedo, I used two devices, a traditional satnav and the Avertinoo "danger spot" device installed on my phone. They both gave the same speed though occasionally differed by 1km/h. Of course that's not to say that they are both right so I have done a bit of additional testing, or what some might call Russian Roulette.
French speed cameras are set with very little room for error. So for example if you are going at a real 97km/h past a camera in a 90km/h zone, you will be flashed. Odds are you will then receive a ticket (assuming they can trace you) for doing 91km/h in a 90km/h zone.
With my newly calibrated speedo I have driven at an indicated 95km/h through a 90km/h camera, and at an indicated 140km/h past a 130km/h camera. In neither case have I been flashed!
Cheers
