Kenlacy, I think you are wrong about the 5MPH. I think you can expect the GPS to be dead nuts on for speed. The 20 meter accuracy would take into account many things, including changes in the ionosphere and other things that can cause it to be off. But, since that is not all changing (for the most part) over the time of a quick race, it will be off by roughly the same amount each time it takes a position reading and calculates speed. If you have been on long enough so you are tracking a fair number of satellites, I think you will find it is right on. Now if you take a position reading one day and mark your spot, and come back the next day and take another one, you could be off by up to 20 meters. But even that is not likely. The only way around that is to use what's called differential GPS. This is what is done at airports and other places. It corrects differences based on the position of a known point, and effectively filters out the unknowns. But for that to work you need to be close to a differential tower sending out the known position, and you need a differential receiver.