That depends. I'm so depressed; brain the size of a planet.
GPS measures effectively in a series of straight lines as I understand it. Speedo measure distance travelled. Rapid acceleration, braking & or bendy roads can throw my GPS speedo but it is bang on vs those flashing speed signs when the speed is constant.
I rely more on the revs as 2k revs = 20 in second , 30 in 3rd, 40 in 4th & 68 in top
