How to calculate the mileage displayed by the odometer and how to adjust the meter of used cars?

Vehicle mileage is an important technical parameter of an automobile, which is closely related to its technical condition, maintenance, remaining service life and scrapping system. For example, the car maintenance cycle is set according to the mileage, 5 thousand or 1 10 thousand kilometers to change the oil, 30 thousand kilometers to change the spark plug, 60 thousand kilometers to change the gearbox oil, tires, and so on; The service life of automobile parts is also closely related to mileage, such as metal fatigue and aging of rubber products. Moreover, China's car scrapping system is also based on mileage, which stipulates that cars will be scrapped when they travel 600 thousand kilometers. ? Therefore, every car owner pays special attention to the mileage of his car, especially when buying a used car. The mileage is an important reference index and even directly affects the price of the car. Do you know how to calculate the mileage of a car? Is it accurate? What factors will affect the mileage of a car? How do used car dealers adjust the odometer? Today we will talk about these issues.

First of all, how to calculate the mileage of a car. Its principle is actually very simple, that is, multiply the number of revolutions of the tire by the circumference of the tire. Because 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 m and the speed is 1000 rpm, then the speed of the car is 1000 m/min and 36 km/h. The main difference between different types of odometers lies in the different ways of obtaining wheel speed signals.

Early cars used mechanical odometers. Its speed signal is obtained from the rear end of the output shaft of the gearbox. The acquisition method is that the output shaft of the gearbox drives the odometer gear to rotate synchronously with the output shaft, and then this gear drives the soft shaft, and the other end of the soft shaft is connected to the odometer to drive the odometer counting gear to rotate according to a predetermined ratio, thus calculating the mileage data of the automobile. At the same time, this flexible shaft also synchronously drives the speedometer to display the driving speed of the car. At present, some motorcycles and agricultural vehicles still use this counting method.

Nowadays, cars basically use electronic odometers. The dashboard is basically a display screen, and the parameters displayed on it are controlled by the driving computer. The speed sensor transmits the wheel speed signal to the driving computer, calculates a value according to the pre-stored program in the driving computer, displays it on the dashboard, and constantly changes with the driving of the car. There are two ways to obtain the signal of the 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.

Whether it is the speed sensor installed on the output shaft of the gearbox or the ABS wheel speed sensor installed on the wheel, their working principles are the same and relatively simple, and they all work by using the principle of electromagnetic induction. A gear ring is installed on the output shaft of the gear box or the wheel, and an electromagnetic inductor is installed above the gear ring. Every gear passing through an electromagnetic inductor will cause a change in the magnetic field. The electromagnetic sensor transmits the changing signal and frequency to the speed sensor, and then the speed of the wheel can be calculated. Then the speed sensor transmits the wheel speed signal to the driving computer, and the speed and mileage display can be seen on the dashboard. ? In some cars, as long as the ABS fuse is pulled out, the odometer and speedometer of the car will not be displayed, that is to say, its speed signal is collected from the speed sensor on the wheel. In order to avoid displaying mileage when moving cars for a long distance, many cars in 4s shops will use this method of pulling ABS fuse to stop the odometer from counting. We can also buy a "zero-kilometer car".

So is the mileage displayed in automobile odometer accurate? Strictly speaking, it is not accurate! The biggest variable comes from the diameter of the tire. When designing a car, the mileage is calculated according to the theoretical diameter of the tire. However, in the actual use of automobiles, tires are often compressed, and the actual diameter is smaller than the theoretical diameter. That is to say, the circumference of the tire is less than the theoretical value for each revolution. For example, in theory, the distance a tire travels in one revolution is 1 m, but the tire is compressed. In fact, the distance traveled by a tire in one turn is 0.998 meters. This will cause the mileage display to be larger than the actual mileage. ? In the national standard, the error of odometer is plus or minus 2%. But in actual use, almost all of them are greater than the actual mileage, which can be clearly seen in the navigation data of the car. Once on a long trip, the last navigation showed that the driving distance was 1025 km, but the mileage shown in the odometer was 1048 km, a difference of 23 km. ? In addition, tire wear, the level of tire pressure and the change of tire model will all affect the mileage calculation. For example, the original car used 205/55R 16 tires and replaced them with 2 15/50R 17 tires. The tire diameter became larger and the driving distance per revolution was extended. Driving the same distance, this 2 15/50R 17 tire shows less mileage. There is also the change of the main reduction ratio of the tail teeth of the rear wheel drive, which will also affect the mileage calculation. For example, if the main reduction ratio of the tail gear decreases, the wheel speed will become faster and the actual mileage will increase. But what's on the dashboard? The mileage is not much.

There is also an interesting question, that is, will it be several kilometers when reversing? I believe most people will say no, and some even think that the odometer will go backwards. Actually, I'm telling you, the odometer counts forward or backward. Because the odometer signal is electromagnetic induction, no matter how the wheel rotates, as long as the magnetic field lines are cut, it will send a signal to the driving computer as the basis for mileage and speed counting. Therefore, when reversing, not only the mileage will be accumulated, but also the speed will be displayed. I have personally tested this point, and the top speed of reversing shows more than 20 km/h.

Automobile mileage is an important technical data of automobile, and it is generally forbidden to change it. All mechanical odometers have anti-reverse devices, and the counting gear can only rotate in one direction, but not in the opposite direction, so as to avoid artificially shortening the mileage. The electronic odometer writes data into the chip, which will not be lost, let alone changed, even in the case of power failure. But we know that after buying a car, used car dealers almost always adjust the odometer and reduce the mileage. How did they do it?

But now the car is more advanced, the dashboard is just a display, and the display content is determined by the driving computer. If you want to change the mileage, you can only change the data inside the driving computer. However, savvy used car dealers are also hard to resist. Someone specially developed the automobile instrument. As long as it is connected to the OBD interface of the car, the mileage of the car can be easily changed. Input as much as you want, which is much more labor-saving than blowing dust. However, some high-end cars, in order to avoid this behavior, will protect mileage parameters, prohibit any behavior of changing data, or only change higher data, such as BMW, Mercedes-Benz, Rolls-Royce and so on. Or it is troublesome to remove the data chip of the driving computer and modify the data on special equipment.

So how do we know if the used car has been adjusted? One is to look at the wear and tear of various parts of the car with rich experience in using the car to see if it is consistent with the mileage; Another method is to check the mileage data stored in the transmission control unit, which is also called "transmission mileage". This parameter generally cannot be changed. If it doesn't match the mileage displayed on the dashboard, you can be sure that the car has been adjusted. However, this method is only applicable to automatic transmission.