What is the GPS accuracy of the speed display compared to car Speedos?

The three major GPS devices – Tomtom, Garmin and Magllan Map 330 – are all known to be more accurate than the car speedometer. The GPS takes the help of 3 meters to track your location. The GPS always gives the most accurate readings even at low speed. On the other hand, the car speedometer has a sensor on the driveshaft to calculate the speed based on the revolution of the wheel. It is very difficult to get an accurate reading from car speedometer. The tires are all made from rubber and there is a variation in diameter with respect to speed and load.

You will be surprised to see the actual difference between a GPS unit and a car speedometer. The mechanical speedometer does not take into account the wear and tear of the tires and the changes in diameter as mentioned earlier. The New speedometers are more accurate as they have a speed sensor which works off the engine but still they are not perfect. However, the certified speedometers give correct readings. There is a strong belief that speedometers are intentionally calibrated at higher than actual speed in almost all the cars. This is the single most important reason why do not get caught speeding so frequently.