Theoretically, GPS can be very accurate; however in practice GPS accuracy can vary depending on variables such as the terrain of the area you’re driving through, the presence of tall buildings, weather conditions, electrical interference and the number of satellites available to the GPS system (a minimum of three, preferably four).
These factors can affect the consistency of the GPS reading, but you are correct in that the GPS-indicated speed is on average, slightly lower than on your vehicle’s speedo.
The Australian Design Rules require that a vehicle’s speedo must not indicate a speed less than the vehicle’s true speed or a speed greater than the vehicle’s true speed by an amount more than 10 per cent plus 4 km/h.
This means that the vehicle’s true speed must not be higher than the speed indicated by the speedo. That is, at a true vehicle speed of 100km/h, the speedo must read between 100km/h and 114km/h. An alternative way to look at it is; at an indicated speed of 100km/h, the vehicle’s true speed must be between 86 km/h and 100km/h. Generally speaking, on modern cars the true speed is only a few km/h less that the indicated speed.
GPS speed readings can be accurate, but not always. It is therefore recommended that you use your vehicle’s speedo for a consistent.