Speedometers vs GPS
Speedometers are composed of many mechanical parts and are therefore prone to mechanical errors.
The speedometers also measure speed by indirectly being in contact with the tires. Thus, a change in any property of the tires results in an inaccurate reading. The most common example of this is a change in the dimension of the tires, followed by tire pressure, normal wear and tear, temperature and load.
Manufacturers calibrate speedometers for the tires installed by them, but when a customer changes the pre-installed tires or uses a set of tires other than the official recommendation, the speedometer needs to be recalibrated, or else the speed reading will be inaccurate.
For safety reasons, manufacturers calibrate speedometers to read higher than the actual speed of the vehicle. A higher value alerts the driver about the speed and prevents him or her from speeding
. However, there is a limit as to how high or low a speedometer is allowed to read. Speedometers are not allowed to read lower than the actual speed of the car, but they are permitted to indicate a value that is 10% higher than the actual speed, plus 4.
GPS, on the other hand, uses a purely mathematical technique to measure the speed of the vehicle and is therefore more accurate than a speedometer. That being said, GPS comes with a catch too. GPS devices work best when the vehicle travels in a straight line and has an uninterrupted view of the sky. A clear view is important, as the GPS receiver needs to interact with satellites
. Also, when the vehicle is traveling on a road with multiple twists and turns, GPS has no means of knowing or predicting how tight of a turn was made. Thus, even GPS readings cannot always
Another shortcoming of a GPS device is that it fails to provide the instantaneous speed of the vehicle; it only gives the average speed, with a slight delay.