I was out riding with friends on Sunday. We got gas and zeroed the odometers on our GPS units. We didn't have 2 of the same model. We rode together for about 200 km and then stopped to talk and checked our GPS odometers. We expected the motorcycle odometers to vary and they did about +/- 7km. The GPS odometers varied about +/- 2km. That is about 1% but I expected better when the predicted accuracy was 3m.
What is your experience? What factors did we miss?