How do car odometers and electricity meters count?

First, let’s talk about how the mileage of a car is calculated. Its principle is actually very simple, that is, multiply the number of revolutions of the tire by the circumference of the tire. Since the circumference of the tire is fixed, we can calculate the mileage and speed of the car as long as we know the number of revolutions of the wheel. For example, if the circumference of the tire is 1 meter and it rotates 1,000 times per minute, then the car will travel 1,000 meters per minute and 36 kilometers per hour. The main difference between various types of odometers is the way in which the wheel speed signal is obtained.

Early cars used mechanical odometers. Its speed signal is obtained from the rear end of the gearbox output shaft. The way to obtain it is to let the output shaft of the gearbox drive an odometer gear to rotate synchronously with the output shaft, and then this gear drives a flexible axis. The other end of the flexible axis is connected to the odometer, and drives the odometer according to a preset ratio. Count the gear rotations so the car's mileage data can be calculated. At the same time, this flexible axis also drives the speedometer to display the car's driving speed. Some motorcycles and agricultural vehicles still use this counting method.

Most of today’s cars use electronic odometers. The instrument panel is basically a display screen, and the parameters displayed on it are controlled by the driving computer. The vehicle speed sensor transmits the wheel speed signal to the trip computer. According to the program pre-stored in the trip computer, a value is calculated, displayed on the instrument panel, and continues to change as the car drives. There are two ways to obtain the signal of the vehicle speed sensor, one is on the output shaft of the gearbox, and the other is to use the signal of the ABS wheel speed sensor.