Auto Questions: How Odometers Work

You can see the numbers on your vehicle's odometer click over, but do you know what makes that happen? Find out how it measures the distance traveled and keeps track of the total number of miles an automobile has traveled.

An odometer can be defined as a "device that measures and keeps track of the distance traveled by a moving vehicle." The idea of the odometer, or the measuring of distance traveled, originally came from the famed inventor named Benjamin Franklin. When he was a postmaster, one of his jobs was to map out mail delivery routes. So, he took his horse and carriage out in order to figure out the best way to plan the different routes. Right away he realized that he needed a way to measure the distance of the carriageways so he could find the shortest ones. The device Franklin invented worked by counting the revolutions of the axles of his carriage.

Today, odometers can be found on every car, truck, van, and motorcycle manufactured today. Many bicycles have odometers mounted on them too. It keeps track of the total number of miles a vehicle has traveled. This is especially important because every vehicle title has a space where the current owner must write in the total numbers of miles the vehicle he/she is selling has on it. Turning back the odometer, or writing in a lower number on the title is fraud, and is punishable by United States law.

So, how does a modern-day odometer work? The odometer is located on the dashboard of a vehicle. It usually is located near the speedometer, and it also shares the same flexible cable if they run mechanically. The only part of the odometer you can see are the six or seven drums. They are all numbered from zero to nine. When the tires of the vehicle turn around, the cable causes the connected plastic gears in the back of the odometer to turn in succession. As each mile is traveled by the vehicle, the gears turn enough so that one (or more) of the drums clicks over to the next higher number. Usually, if the vehicle has traveled one mile, the next to the last drum to the far right clicks up a number. If, however, the vehicle already has an odometer reading of eighty-nine thousand, nine hundred and ninety-nine miles, when another mile is driven, all five of the drums will click over to the next number. This, of course, will make the new odometer reading ninety-thousand miles.



The drum that is the farthest to the right is called the "tenths" counter. It adds up every tenth of a mile that a vehicle travels. When it reaches ten tenths of a mile, of course, it clicks back to zero, and the number in the next column clicks up a number higher.

There is a second type of odometer on a vehicle that is called a "trip odometer." Unlike a mileage odometer, this odometer can be readily set back to zero with the simple push of a button. It's not for measuring and keeping track of total miles put on a vehicle. Its purpose, just like its name implies, is to simply keep track of the number of miles that a certain trip from point A to point B used.

This type of odometer can be especially useful for checking to see how many miles per gallon you're getting on a tank of gasoline used by a vehicle. You can fill the gasoline tank up in your vehicle, reset the trip odometer, and drive the vehicle until it almost runs out of gasoline. Or, you can fill up the tank, reset the trip odometer, and watch for when you have driven enough to use up, let's say, a quarter of a tank of gasoline. If the tank holds twenty gallons, and you use a quarter of a tank, that means you have used five gallons of gasoline. If you have driven one hundred and fifty miles, according to the trip odometer, that means that your vehicle got thirty miles per gallon of gasoline.

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