On a flat straight road in clear conditions where the system can hook up to the satellites, GPS will be more accurate. On twisty, hilly roads it won't be because it measures your speed as a crow flies, across the surface of the planet. Also, anywhere it can’t pick up the satellites due to tall buildings, for example, or in a tunnel, it will compute your speed and could make a significant error. (Thanks to Peter Rodger, Chief Examiner of the IAM, for this informatioin.)