The odometer is one of the most critical sources of information for car owners, second-hand car buyers and dealers, and fleet managers. While most people only notice the displayed data, there’s much more to the odometer. This article is all about that – understanding how odometers work.

What is an odometer?

An odometer is a device used for measuring the distance covered by an automobile. It can be mechanical, electronic or a hybrid of both technologies – electromechanical. These instruments have been around for a very long time with early forms tracing back their origins to ancient China and the Greco-Roman era.

The odometer is sometimes called an odograph or mileometer (or milometer), especially in countries that have adopted US customary or imperial units.

How accurate are car odometer readings?

The distance recorded by a vehicle’s odometer increases over time as it makes more trips. Several calculations go on under the hood of your car to maintain an accurate odometer. But on rare instances, such as when the engine is replaced without recalibrating the on-board computer, the odometer may lose its accuracy. The computer will keep calculating based on the original engine, and this translates to inaccurate odometer readings.

It is also important to note that GPS is a line-of-sight technology and can affect the odometer reading. This can be the case when you only use GPS to track your odometer. There are situations where GPS connection has to be re-established, such as when parking underground or when you drive through tunnels. 

When this happens, chances are your odometer will record less distance than what your vehicle travelled in reality.

How do odometers calculate distance travelled?

Today, most odometers work by recording wheel rotations. The distance travelled is then calculated by multiplying the number of wheel rotations by the tyre circumference. The circumference of a tyre (circle) is obtained by multiplying the tyre diameter by pi (3.1416).

Non-standard and severely worn-out tyres corrupt the odometer reading as they introduce the errors. The actual distance travelled is calculated by the simple formula below:

Actual distance traveled = [(Final odometer reading) – (initial odometer reading)] * (actual tyre diameter) / (standard tyre diameter). 

However, understanding how odometers work, goes beyond this simple formula. Odometers are sometimes off, but that's normal as no system is flawless. Odometer errors are proportional to those of the speedometer.

How do odometers work?

Throughout the years, cars have evolved in terms of design, efficiency, and technology. However, the odometer operation has not witnessed any significant changes. Cars still employ either of the two approaches to track mileage.

Mechanical odometers

The operation of mechanical odometers starts at the transmission. Here, a small gear used for advancing the odometer is fitted into the transmission system. The speedometer drive cable is then connected to this small gear. The other end of the cable connects to the instrument cluster.

When the engine is turned on, and the car starts moving, the internal transmission gear turns. The connected drive cable conveys this internal transmission gear motion to a set of gears linked to changeable digits.

Counting starts at the right side of the set of numbers. The process [counting] goes on until the amount of distance travelled by the vehicle forces the adjacent digit on the left to roll over. The process repeats itself until all adjacent numbers reach their peak value. At this point, they all reset to zero and start over again. It is not uncommon for the digits on a mechanical odometer to be slightly out of line. 

Electronic odometers

Electronic odometers came after the mechanical ones and are typically digital. As the name suggests, these rely on the vehicle's electronics to establish accurate mileage. Much like mechanical odometers, electronic ones still employ a special gear to change the count reflected on the dashboard. 

In place of the drive cable, however, a magnetic sensor is used to track the number of gear turns in the transmission. The signal obtained is then conducted by wires to the on-board car computer which interprets and converts the information into a mileage count.

Electronic odometers have the edge over mechanical ones due to their better accuracy. Since they aren't easily manipulated, they generally give an accurate count of the vehicle's mileage. Digital electronic odometers make car buying safe for potential car owners considering second-hand vehicles.

It is worth noting that odometers come with an additional trip meter known as a trip odometer. This trip meter helps car owners determine the mileage of any particular distance without checking the main odometer.

Is it possible to change the odometer reading?

Wily car dealers can alter the odometer reading to charge unsuspecting clients more. By resetting or rolling back the mileage, a dealer can convince the car buyer that the car has lower mileage than it actually has. 

How do you tell if the odometer has been rolled back?

Here are some tips to avoid falling victims to odometer fraud.

• If the car uses a mechanical odometer, check and listen for odometer clicking. 
• Inspect the inside of the clear plastic lens cluster cover for any fingerprint marks.
• Check out the maintenance history of the vehicle.
• Use car history websites in Australia (such as CarFacts and CarHistory) to find out facts and reports about the car, including reported odometer readings.
• Ask a trained technician to inspect the car and estimate its mileage and valuation.

Why is the vehicle odometer important?

The primary purpose of an odometer is to indicate the distance travelled by a vehicle. However, it has other secondary purposes depending on the context. For instance, it is used to represents milestones for scheduled maintenance such as oil changes, tyre rotations, and comprehensive vehicle inspections in fleets and personal cars. Odometer readings are also used by dealers in the used car market to estimate the valuation of a vehicle. For this reason, understanding how odometers work should also be your concern as a car owner. 

Learn more interesting things about your car from Carpart.com.au. We publish blogs daily, so subscribe now to get updates! We’re also your quickest link to auto parts sellers wherever you are in Australia – how does this work? Just send us a request for the part you need, and we’ll send it to over 500 sellers. In a matter of a few hours, you’ll start receiving quotes for the exact part you want. Beats endless searching! 

By Sam O.