The commercial GPS units have the same resolution as the military units, ever since they removed "selective availability". The GPS may only be accurate to x distance but the accuracy is not random, it doesn't rapidly jump all over the place from sample to sample. It does drift over time though, so you will notice if you ride in a stationary position for an hour you may get a hundred feet or so of "movement" detected. You have a far higher chance of your tire circumference being off enough to accumulate more error than with the GPS.