Another problem with GPS is the accuracy (or lack thereof). Sometimes you get a good GPS signal (e.g. outside, clear view of the sky etc.), sometimes you don't, and you position could be off by 10m. Some GPS store the accuracy of each track point (e.g. VDOP and HDOP), some don't. If your GPS doesn't store the accuracy, then someone could claim your GPS is off by 20m, and hence you were speding (or the data is unreliable).