Speedometers are allowed to overstate speed by up to 10 per cent, but never to understate it. Speedos therefore often indicate a higher speed than GPS. At a steady speed on a straight road, sat-nav is likely to be more accurate than the speedo.
GPS devices calculate speed by determining the time taken to travel a given distance, so bendy roads or rapid acceleration/deceleration distorts the readings. There’s also a short time lag while the GPS calculations readjust.