English 
a speedometer normally measures
The speedometer indicates the speed at which the vehicle is traveling, it shows the instantaneous speed. The indication of the speedometer depends on the vehicle speed sensor (rear wheel driving wheel). The sensor is installed on the gearbox. Through different speeds, different pulse signals are obtained, and then converted into pointer output, which is the current speed you see.
A speedometer is a gauge on a car's dashboard that measures a vehicle's speed. It usually consists of a pointer and a dial, the pointer points to the number on the dial, showing the current speed of the vehicle. The working principle of the speedometer is to convert the mechanical energy generated by the rotation of the wheels into an electrical signal, and convert it into a visual speed value. The key component in a speedometer is the speed sensor. A speed sensor is an electromechanical device that senses how fast a vehicle's tires are turning. When the vehicle is moving, the magnet in the sensor generates an electrical signal in response to the rotation of the wheel. This electrical signal is then converted into a digital signal and passed on to the vehicle's control system. The control system decodes these digital signals into vehicle speed, which is then displayed on the speedometer.
According to the installation position and principle, the speed sensor is divided into magnetic sensor, hall sensor, linear magnetic sensor, gear sensor and so on. Among them, the gear sensor is the most commonly used. It uses a combination of a rack and pinion to measure the number of rotations when the wheel is rotated. Every time it rotates a certain number of times, a certain electrical signal will be generated, and the control system will decode these electrical signals into vehicle speed. When using the speedometer at ordinary times, it should be noted that the error of the speedometer is very small, but it cannot be completely adjusted to an accurate value. The error of the current speedometer is generally between 5% and 10%. In addition, when the vehicle is driving, the speedometer will be affected by many factors, such as road conditions, tire pressure, vehicle load, etc., which will cause the error of the speedometer to change, so we should comprehensively consider the speedometer readings and Other factors can estimate the speed of the vehicle more accurately to ensure safe driving.
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