What is the odometer in a car

What is the odometer?

The driver constantly sees the dashboard in front of him while driving a car, on which various measuring devices are placed. Thus, the speedometer displays the current speed of movement, the tachometer shows how many revolutions per minute the crankshaft makes. There are also indicators of oil pressure, battery charge, and the temperature of fuel. On trucks and passenger vehicles, there are gauges that show brake system pressure, tire pressure, and transmission oil temperature gauges.

There is also another gauge, usually located between the tachometer and speedometer, that shows the miles driven. This device is called an odometer – a very useful thing. In particular, if you buy a car with mileage, you need to check if the mileage was not twisted. How to do this – we told earlier on Vodi.su in a previous article.

Principle of operation

Knowing the radius of the wheel and the speed of the car, you can use a simple formula to determine the angular velocity at which an arbitrarily selected point on the circle moves around the center. Well, operating with all this data, you can easily determine how far the car, cart or chariot has traveled.

Indeed, the idea for this simple device came to the mind of the Greek mathematician Heron of Alexandria, who lived in the first century AD. According to other sources, the first person to get the idea of the odometer was either the well-known Archimedes or the Chinese philosopher and thinker Zhang Heng. In any case, it is known for certain that already in the III century AD the Chinese were actively using this invention to measure the distance traveled. It was called their “counter of the way, which passed the cart.

Today, the device is installed on any car and motorcycle. It works according to a simple principle: the counter is connected through a sensor to the wheel. The sensor determines the angular speed of rotation, and the CPU calculates the mileage traveled.

The odometer can be:

  • mechanical – the simplest option;
  • electromechanical;
  • electronic.

If you have a more or less modern car, most likely it is an electronic odometer, which measures the distance traveled due to the Hall effect. We also previously wrote on Vodi.su about Hall sensor, which measures directly the speed of the crankshaft. The data obtained is absolutely accurate, and the error in measurement is minimal, not exceeding 2 percent (for electronic) and five percent (for mechanical and electromechanical devices).

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What do I need to know about odometers?

The advantage of electronic odometers over less advanced types is that the electronic odometer does not reset. In a mechanical indicator, on the other hand, the wheels make a complete circle and are reset to zero. As a rule, kilometers over 999 thousand kilometers are not displayed. Basically, few vehicles, except cargo vans or passenger buses, are capable of covering such a distance during their entire service life.

It should also be noted that the odometer displays both the total kilometrage and the distance traveled over a certain period of time. This applies to both electronic and mechanical odometers. Typically, the indicator is located directly on the dial of the speedometer. Therefore, it is probably often thought that the speedometer and the odometer are one and the same instrument. The upper window shows the total kilometrage, while the lower window shows the distance traveled during the day. These readings can be easily reset.

When buying used cars, the first thing drivers do is check the mileage that the odometer shows. There are a number of signs, which can be used to guess that the mileage was twisted on the mechanical odometer. In principle, bad masters have learned how to twist and electronic devices. But it is necessary to understand that in modern cars all data about the condition of the vehicle is stored in the computer memory, and it is almost impossible to clean it. That is why if you have any suspicions, you should either refuse to buy a car, or take it in for full diagnostics to find out its real mileage.

What is the odometer and what is it for?

What is an odometer, types of the device, the principle of operation, how to find out about the twisted odometer. Video about the odometer.

What is the odometer and what does it do?

If you ask what is the odometer in a car, it is not the fact that everyone who is asked this question will answer it. Although everyone is well aware of what the speedometer is. Even looking at the readings of this device, you can not guess that there is an odometer in front of your eyes. This is not surprising, because it is located on the panel along with the speedometer, so it is easy to combine these two different devices into one.

What is this device – the odometer

What is an odometer?

What data the device shows, everyone knows. It’s just a mileage meter, which counts the number of revolutions made by the wheel while driving.

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Often people think that the speedometer displays along with the speed of the car and its mileage. But this is a misconception, because the odometer scale is simply compactly integrated next to the speedometer scale – this is done for the convenience of the driver and to save space. But if you look at the instruments “from the inside”, taking off the cover of the panel, it becomes clear that they are completely different devices.

How the odometer works

To find out what the distance traveled by a wheeled vehicle, the formula for determining the angular velocity of a certain point on the circumference of the wheel is used. Other parameters for the calculation are vehicle speed and wheel radius. All this is implemented in the work of a simple device – the odometer.

This device is known to mankind since time immemorial, it was invented by the ancient Greeks or Chinese – exactly not known. The modern odometer is no longer a primitive construction, but a structure consisting of a rotational angular speed sensor and a CPU that calculates the kilometers traveled.

Types of odometers

Types of odometers

The first odometers were mechanical, but modern cars are equipped with electronic devices. There is also an intermediate option – electromechanical odometers. What is the peculiarity of their work?

The mechanical device has a cable connected to the dashboard and the shaft of the box. The rotation is transmitted through the shaft – the revolution turns the gear by a tooth. The mechanical odometer has five rotating drums, where the numbers are placed, which show the mileage, gradually turning.

The electronic odometer has a toothed gear on the output shaft of the box. When the gear and cog pass by, the meter counts pulses. The computer calculates the wheel revolutions, displaying this information on the display. The travel between pulses is calculated, and the mileage information is updated, from the ECU going through the electronic circuits.

The computer is more difficult to adjust because the ECU is able to not miss and account for all pulses.

The odometer has two mechanisms. The first one displays the total mileage of the car since it came off the factory assembly line, which cannot be reset to zero. At the bottom there is another indicator reflecting the daily mileage, and it can always be reset to zero.

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What the driver needs the odometer for

A small counting device plays an important role in the functioning of the car and is significant for the car owner himself.

Firstly, it is always useful to know what is the real mileage of the car, so as not to be mistaken about your own vehicle. In addition, it is possible to determine the distance from point A to point B using the odometer.

Secondly, guided by the readings of this device, the owner can easily find out when it is time to carry out maintenance, when to change the oil – these actions depend on a certain amount of mileage. This also includes such elements of car servicing, as adjustment of valves, camber and convergence adjustment, replacement of timing belt.

Third, the daily mileage meter can help determine the exact fuel consumption. If the driver suspects that the consumption has increased, it is possible to calculate the fuel consumption before going to a service station and checking the engine. To do it one should have a canister of gasoline, and when the fuel in the tank is completely burnt out, one should reset the bottom (daily) counter to zero.

After that, the tank is filled with fuel from the canister stocked and continue to drive until this gasoline is also used up. Then the volume of the canister in liters divided by the mileage according to the readings of the daily meter, multiplying the received number by 100.

The problem with twisting the odometer

The problem of twisted odometer

The total mileage of a car is an important parameter, which in the first place attracts attention at sale or purchase of a handmade car. That is why there are car owners who try to reduce the odometer reading – twist it by certain means.

The first thing a customer will do is to check the figures on the dashboard display, and based on what he or she sees, make a conclusion about the condition of the car parts and components – engine and running gear. A car with high mileage loses a lot in the evaluation of its value, and it is much harder to sell it.

Mileage is important not only when buying or selling a car. It is also important for commercial use of the car, in which case it is necessary to monitor the vehicle’s non-operational use and fuel consumption.

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Unscrupulous drivers, in order to hide the misuse of their cars for “illegal” earnings or personal purposes, also try to twist the odometer, so that the mileage figure is decreased. How do they do it?

We should not underestimate the skilled craftsmen, capable of tweaking even the electronic systems for twisting kilometrage. In our country, this action is not punished in any way, but in the countries of Europe, interference in the readings of meters may be punished with further stay in the room with squared windows and for a long time.

Mechanical odometers on older cars are twisted easier than other types of meters. To do this, “skillful hands” remove the cable from the shaft and connect it to a rotating power tool like a drill, making the rotation in the opposite direction. Sometimes you have to remove the counter and spin the drums manually.

To correct the electronic odometer, you will need software and special devices, which give the opportunity to infiltrate the onboard computer and change the readings there. Sometimes the onboard controllers are reprogrammed.

How to detect a wire twist

How to Detect Odometer Twist

From the point of view of those who underestimate the mileage of the car, they do nothing wrong. As a rule, it happens before the sale, to get a higher amount of money for the car. But in fact this act is a common fraud, deceiving the buyer, who after purchasing a vehicle with good mileage will soon face the old and worn out parts that need repair. He will also face untimely maintenance, improper diagnostics and many other hassles.

A driver who doesn’t have much experience may not notice signs of tampering with the odometer reading. But a professional mechanic will always determine if the odometer was twisted – for this purpose, it is inspected on the pit. In case of mechanical odometer he will see that the place where the cable connects to the gear at the box is suspiciously clean, although all other elements will be covered with dust and dirt. It means that the cable was removed for a specific purpose.

If an electronic-mechanical device is checked, look at the place where the body of the gauge is attached to the panel. If it was removed and the gears were disconnected in order to set other values, there will be noticeable interference. The impact on the screw fasteners is also evident.

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If the interference was in the electronic odometer, it can be noticed if the dashboard was disassembled. In other cases, it is practically impossible to notice the re-flashing. Then check the overall condition of all the nodes, comparing them with the odometer readings and draw conclusions.

How accurate is the odometer reading

How accurate is odometer reading

Absolute accuracy of the odometer will not show, as well as all other measuring instruments in the car – for example, speedometer or tachometer. All of them give out a certain margin of error. For a mechanical odometer, this error is set at 5% to 15%. As the speed of the car increases, the inaccuracy also increases.

Even more inaccuracy will be shown by the device, if the car is often skidding, when its wheels are rotating idly. In the case of slipping, even the electronic odometer will not reflect the real mileage – because the wheels will work in one place, and the counter will increase the mileage, as it counts wheel revolutions, but not the distance itself.

There will also be inaccuracy if the car owner installs wheels of the wrong diameter. The counter will wind up more kilometers if the wheels have a smaller diameter, and vice versa, if the wheels have a larger diameter.

In any case, the most accurate are electronic odometers, there may be an error only when the tires are worn. If the counter is a little “tired” (all subject to wear) and therefore does not give reliable information, it needs to be repaired without putting it off for a long time, so as not to run into problems with the future servicing of the car or when selling it.


The odometer plays a great role in maintaining your vehicle, allowing you to do it on time at the right intervals. In the process of personal evaluation of a car when buying or selling, the odometer shows the objective mileage (if it is not twisted!) and enables an objective evaluation of the vehicle.

With the odometer you can calculate the amount of fuel consumed and make sure that the engine is running properly, or that is not the case. It also helps to calculate the distance between two points when the vehicle is in motion.

The odometer is not part of the speedometer, although its display is adjacent. The odometer, although small, is an important car measuring device.

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