I understood even the GPS indicated speed is an extrapolated speed rather than genuine speed. Most GPS units do not, by default, update gps position more frequently than once per second which is not accurate enough for 0-60 timings. They would allow you to see how inaccurate the cars speedo is when maintaining a steady speed and course eg on a motorway. Even if you know the cars speedo is indicating 60mph accurately at a steady speed, there is no way to allow for the lag of the needle travelling around the dial when accelerating. This is why most Magazines use additional external equipment to measure speed and acceleration.