My comments about the accuracy of GPS mostly revolve not around the accuracy of its positioning, but how speed is calculated. Speed is difference in distance divided by difference in time. While I do not own a GPS, I believe GPS locations are accurate within something like 50 feet, which is very accurate, and differences in location would have the same accuracy. However, I don't know what the sampling frequency is for the calculation by the GPS. If the calculation is performed every tenth of a second, it might not be totally accurate. If it's averaged over, say, every ten seconds, it might be extremely accurate.
If you have a GPS and are driving at a constant rate of speed, and the GPS reading of your speed does not vary (within 1 mph), then I would assume that it is accurate to a precision of less than 1 mph, so it is not an issue.