Report: Your car's odometer is deceiving you about the actual distance you've driven

Bethlehem - Ofoq News - The odometer in your car seems to be one of the simplest devices in the vehicle.

For example, it indicates that you have driven 42,000 kilometres, so you might assume that the car somehow knows the exact distance you have traveled. But the truth is, there is no little measuring wheel rolling on the road under your car for this purpose.

Instead, the odometer usually calculates the distance based on information about the vehicle's movement. Depending on the type of vehicle, this information may come from wheel speed sensors or the vehicle speed sensor, as the car's electronic systems use programmed standards to convert those signals into miles or kilometers.

Wheel speed information can be used to calculate cumulative distance, and factors such as “tire rolling radius” and “number of sensor ring teeth” are parameters used in this process. Here's how it works:

Your car actually counts the number of wheel revolutions

Think of it this way: If the computer knew the approximate distance the car traveled each time the wheels completed a full turn, it could simply add those smaller distances together.

Old mechanical odometers achieved the same basic result using gears and cables, while modern vehicles do this electronically. Sensors detect rotation, the car's computers process the signals, and the calculated result is then sent to the dashboard or stored elsewhere inside the car.

This means that the odometer does not need to know your geographical location, it does not need GPS coordinates or maps. Rather, it can calculate the cumulative distance simply by tracking the rotation of the car's wheels or transmission system.

Tire size matters more than you might think

Here's the interesting thing: the size of your tires can affect the calculation, as a larger tire travels longer per lap than a smaller tire. If your vehicle's electronic systems are adjusted (calibrated) for a particular tire size, and you install tires with a significantly different rolling circumference, the relationship between wheel spin and actual distance traveled will change.

Vehicle maintenance documentation provided by the National Highway Traffic Safety Administration (NHTSA) explains why manufacturers should consider this. A technical bulletin issued by General Motors (GM) instructs technicians, when checking odometer accuracy, to ensure that the vehicle is equipped with the correct tire and wheel size, and that the vehicle calibration settings match the tire size and axle ratio.

For this reason too, changing tire size may affect more than just the accuracy of the odometer, as vehicle speed calculations can also be affected, since the car relies on rotational data to estimate its speed while driving.

Here's why the odometer is never completely accurate

The odometer does not measure actual "kilometres" on the road with laboratory accuracy, but rather calculates the distance based on the vehicle's calibration settings. There are several factors that may affect this calculation:

Tire size: Different turning circumference changes the distance the car travels per wheel revolution.

Tire Wear: As a tire wears, its actual rolling circumference decreases slightly.

Tire Pressure: Changes in inflation level (pressure) can change the tire's turning radius under the influence of the vehicle's weight (actual turning radius).

Wheel Speed ​​Signals: Sensors and signal loops provide the rotation data on which the system relies.

Vehicle Calibration: The vehicle's computer must have accurate information about tire and transmission system specifications.

GM maintenance documentation indicates that dealers can verify the accuracy of the odometer by comparing its reading to portable GPS data while driving on a known-distance route. The documents also explicitly warn against simply comparing one vehicle's odometer reading with another vehicle's odometer reading.

So, when the odometer indicates that you have traveled exactly one kilometre, that number is actually the product of a calculation, and not a direct measurement of the actual distance of the road you are on.

Odometers usually do not depend on GPS: This is perhaps the most common misconception.

Your car may use GPS for navigation, location services, and other functions, but the odometer does not need this system to calculate the distance traveled, as it can continue to record the distance based on wheel speed or vehicle speed data, even if a GPS signal is not available.

This makes sense when considering how the system is designed and how it works. A car needs to know how far it has traveled, regardless of whether it is traveling inside a tunnel, in an underground garage, or in a place that lacks a reliable satellite signal.

In its regulatory interpretations, the National Highway Traffic Safety Administration (NHTSA) also distinguishes between an odometer and a speedometer, noting that the odometer can display the distance traveled in either miles or kilometers, depending on the type of vehicle.

Drivers wonder: How does your car's odometer know how far you've traveled?

This question has often been asked online: If a car doesn't directly measure the distance on the road, how does the odometer know that?

And here lies the excitement when talking about changing tires. In a discussion that took place in 2021 on the “Reddit” platform (in the r/cars section), a user asked whether changing the size of wheels and tires might lead to recording the distance traveled (which the odometer reads) at a faster or slower speed. Commentators have explained that the decisive factor is the overall diameter or circumference of the tire, as a different circumference means that the car travels a different distance with each rotation of the wheel.

Your car counts every kilometer you travel

An odometer may seem like a simple counting device, but there is an intelligent process going on behind that number.

The vehicle detects the turn, applies its own calibration settings (such as tire size), and then continually adds the calculated distance to the total distance traveled, which is why something that seems simple and straightforward — like changing tire dimensions — can affect the odometer reading.

So, next time you look at your car's odometer, remember: Your car isn't actually measuring the road, it's performing calculations.