First of all, it is necessary to understand the working principle of GPS speed measurement and speedometer: After receiving the radio waves of more than 4 satellites and above, the GPS terminal can determine the position of the vehicle and calculate the driving time through the change of position in a very short time. The distance is obtained to obtain the driving speed; the signal of the speedometer comes from the speed sensor on the gearbox or the output shaft, which is synchronized with the rotational speed of the wheel. After the signal is processed, it is sent to the stepping motor pointed by the control pointer in the speedometer to display the current speed to the user.
According to the principle of the speedometer, there are two main reasons for the difference between the speedometer and the actual speed:
The tire pressure of the automobile tire is insufficient or due to wear during use.Its radius gradually decreases. The speedometer's signal is synchronized with the wheel speed and it is not known that the wheel radius has changed. Therefore, under the same actual speed, the speedometer indicates that the speed is higher than in the past.
The stepping motor in the speedometer is faulty and there is a lost step.
Compared with the speedometer, the GPS speed measurement has fewer interference factors, and the accuracy and stability are more guaranteed, but slightly delayed. In fact, the speedometer speed is often slightly higher than the GPS speed. This is because the national standard "Safety Technical Conditions for Motor Vehicle Operation" (GB7258-2004), 4.12 specifies the range of indication error of the speedometer (unit: KM/H): 0 ≤ (indicating vehicle speed – actual vehicle speed) ≤ (actual vehicle speed/10 + 4). Therefore, as long as it is a qualified speedometer, the display speed is not lower than the actual speed; and as the tire wears, the display speed is higher than the actual speed.
Reminder: Some owners have a certain misunderstanding of the regulations, thinking that the speedometer is at 70KM/h, the actual speed is 60KM/h; when the speedometer speed is higher than 70KM/h, it can overspeed 10KM/h without worrying about being punished when measuring speed . This is only the upper limit of the standard. It is also possible that the speedometer and the actual speed are not much different. Be careful not to overspeed.
When your speedometer and GPS speed are quite different, you should use other GPS equipment to confirm whether the speedometer is displayed incorrectly. If the speedometer displays incorrectly, you should first check the tire pressure. When the tire pressure is normal, you need to go to the franchise store. Check and repair.