Most phones can get a GPS lock even with flight mode activated. It uses data network only to get a GPS lock in lesser time. All GPS locks are primarily directly through satellite. 3G or 2G has no say in how accurate your GPS lock will be. It depends on the sensitivity of the GPS Antennae on your device. So, for best results, switch to flight mode, wait for lock (could take 2-5 minutes depending upon device) and then use a GPS tracker to track your speeds while you concentrate on the road.