Is the speedometer on the car accurate?
Many drivers wonder when they see that the speedometer on their car is inconsistent with other measuring devices.
How a speedometer works
The vehicle's speedometer is used to indicate the distance the vehicle travels and the time it takes to complete that distance. However, this is only a way to calculate average speed and does not apply to calculating the speed of a car. For vehicles such as motorbikes and cars, the speed measuring system will measure the number of revolutions of the drive shaft with a speed sensor.
During the use of the vehicle, if the user changes the wheel, the tire is larger, this measurement will no longer be accurate. On the contrary, if the tire is smaller, the vehicle will travel a shorter distance for each tire rotation, the vehicle will go slower.
Therefore, even a few millimeters difference in the diameter of the wheel will cause an error that will cause the speedometer to measure incorrectly. If the car is traveling at a speed of about 50 km/h, the wheel will rotate about 6-7 times per second, so it will make a big difference. This margin of error has been taken into account by the regulations as well as the car manufacturer when setting up the speed sensor.
How a speedometer using satellite positioning works
The satellite positioning system is used to calculate the speed of the vehicle by measuring the distance traveled and the time according to the information obtained from the GPS system. The feature of this system is to continuously determine the exact location of the user on the earth through satellites and calculate how far the vehicle has traveled, then divide by the time the user traveled that distance.
In terms of accuracy, the system is determined to ensure that the signal and satellite quality are not related to the car's tires.
However, the limitation of this system is that it can only calculate a small number of speeds when moving in a straight line, not accurate for the distance uphill or downhill. The satellite speedometer will give a lower error when the car is operating at high speed, because the larger distance over time reduces the rounding error. Therefore, the satellite speedometer will usually display the car's real speed more accurately than the speedometer.
