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Before the days of GPS, there was a time when drivers had to print out directions to their location before leaving their house. And before that, without the internet at all, they had to bring along a map to find their way to their destination (gasp!).  With both modes of directions, it was necessary to keep track of the miles you were driving in order to find the turns you needed to make. Now, with the invention of the Global Positioning System, your smartphone or standalone GPS device tells you exactly where and when to turn without you having to track the miles you are traveling.

Do you ever wonder how cars measure the distance that they travel?  Although it seems second-nature to look down on your dashboard and see how many miles or kilometers you have traveled, the process behind the numbers is a little more complex than you would think.  More than likely, you have not thought twice about how the numbers are calculated.

The Odometer

The odometer is the instrument in your car that measures the distance traveled.  While all odometers do the same thing, not all of them do it in the same exact way.  Some odometers can be electronic, or computerized, while others are mechanical. These odometers work in different ways to measure the distance that your car travels.

Mechanical Odometer

The mechanical odometer has been around for centuries.  Believe it or not, they were first invented in the 1600’s to measure the distance that wagons or horse-drawn carts were going.  Since then, mechanical odometers, or versions of them, have been invented for boats, bicycles, and all other types of vehicles.

A mechanical odometer is made up of a series of flexible cables, springs, and gears that work together to slowly turn the numbers.  In a car, one of the gears engages the output shaft of the transmission, turning a cable that then turns another gear. More gears and cables work together to slowly turn the numbers based on the number of tire rotations. Because of this system, one can run the car in reverse to make the numbers decrease and remove miles from a mechanical odometer.  In an even easier fashion, one could also hook a drill up to the cable and remove miles that way as well.

Mechanical odometers are now far and few between.  They are not actively made in cars anymore as technology has advanced.  Mechanical odometers were generally phased out of modern automobiles starting in the early 2000’s.  You could still find a mechanical odometer if you collect old cars or own a car made before the early 2000’s.

Electronic Odometer

After the mechanical odometer came the electronic or computerized odometer.  This is the type of odometer used in most cars today. Modern cars are made up of dozens of tiny computers that work together.  These computers do everything from raise and lower your windows, to notify you of someone or something in your blind spot. One of these computers has the job of counting rotations of your wheels.  It knows that a certain number of tire rotations equals one mile. When that number happens, the computer changes the number of miles on your odometer.

There is math involved with an electric odometer as well.  Just like calculating the circumference of a circle, the odometer needs to take into consideration the circumference of the tires of the car.  This calculation involves the constant, pi (π). The formula for the circumference of a circle is 2πr, where r is the radius of the circle. So, the computer uses this formula to calculate how many tire rotations equals one mile.

Because the circumference of your tires is involved in the calculation of your odometer reading, it is important to keep the same type of tire on your car.  Putting different tires on your car can alter the accuracy of your odometer reading as the computer in your car is programmed to use the circumference of a specific tire.  If you do need to put a different type of tire on your car, make sure to ask your dealership if it is possible to reprogram your computer for a different type of tire.

Trip Odometer

The trip odometer is another type of odometer that some vehicles have.  This odometer measures the distance that the car travels, however, it is able to be reset back to zero.  This reading does not alter the number of miles registered on the regular odometer for your car. It is a separate reading that allows you to start from zero to measure how long a certain driving distance is.

Why is odometer accuracy important?

All of this information might be a little overwhelming, but it still doesn’t answer the question as to why it is important for the odometer reading to be accurate.  Does it really matter? Is it really necessary to know the exact amount of miles you have traveled? An accurate odometer reading is important for a number of reasons.

The first reason an accurate odometer reading is important is for vehicle maintenance.  Most car dealerships and auto bodies measure the distance a car travels to know when it is time for certain car maintenance to be performed.  For example, an oil change for a car typically happens after about 3,000 miles have been traveled. Most car manufacturers also recommend a general tune-up after 30,000 miles have been driven.  Properly maintaining your vehicle is essential for a car that is reliable for years to come.

Another reason that an accurate odometer reading is important is when it comes time to sell your vehicle.  No matter where you decide to sell your car or who you decide to sell your car to, they will want to know the odometer reading.  The number of miles on your car is important in learning the resale value of your vehicle. Typically, the more miles on your car, the less value it holds.  It is actually illegal to tamper with your odometer reading for this reason.

Who knew there was so much behind those little numbers on your dashboard?  We tend to take for granted the small technologies that go unnoticed in our lives, like the odometer.  So, the next time you get into your car, take a glance at the numbers on your dashboard. How long has it been since your last oil change?  How many miles has your car traveled since your purchase? Keeping track of the information that your odometer provides will make you a more responsible car owner.  It will allow you to properly maintain your vehicle with tune-ups and oil changes at the appropriate milestones, as well as sell your vehicle for a reasonable amount.  Pay attention to what your odometer is telling you, and your car will thank you for it!

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