An odometer is a tool of measurement used to determine how far a vehicle has traveled. It works by use of a small gear driven wheel that runs off of the output shaft of the transmission. The number of rotations of that shaft is the same as the number of rotations of the wheels. Since the output shaft spins the same as the wheels, and accuracy of the measurement won't suffer, they put the gear driven wheel in the transmission. They do this because it is less likely that the small wheel will break inside the transmission than near the road and wheels where rocks may crack or break the wheel.
The distance traveled is calculated by multiplying the diameter of the tire by pi (3.14159) to find the circumference of the tire. The circumference is then multiplied by the number of rotations of the output shaft which is the same as the number of rotations of the tires!
That math for the odometer is done in the lab where the vehicle is being designed. The vehicle doesn't actually do this math. The small gear driven wheel is designed to be the correct diameter in relation to the diameter of the output shaft so that it doesn't need any calculation, but simply to count.
The older mechanical style odometers, like the one pictured above, use numbered wheels with 0-9 on each of the wheels. The numbers count up from right to left. The far most right wheel will count from 0 to 1 to 2... on up to 9 and then when it goes back around to 0 the wheel to the left of it will turn from 0 to 1. When the first wheel gets to 0 again, then wheel to the left will turn to 2. That pattern is followed all the way out to all the digits.
The furthest wheel to the right is the 1/10 wheel. It tells you how many tenths of one mile you have gone. If there is a 1/10 wheel it is colored white or a different color from the rest, or it is highlighted, or it follows a decimal point on the newer digital odometers. If that wheel looks identical to the rest of the wheels then there is no 1/10 wheel and it is just the ones place.
The places go from right to left like this: tenths place, ones place, tens place, hundreds place, thousands place, ten-thousands place, and on newer vehicles hundred-thousands place.
Older vehicles had no use for a hundred-thousands place in the odometer because the vehicles weren't expected to last that long without major work. This was do to many things including looser machining tolerances in parts like the valve and cylinder bores and throughout the whole engine, lesser quality lubricants, and that materials engineers had not discovered all the great materials used in modern vehicles.
Today's vehicles use digital odometers. They work just the same as the analog odometers, but use a neat little screen instead of the numbered wheels. They have the 1/10 place after a decimal instead of using a differently colored wheel.
Most vehicles also have what is called a trip odometer, or simply a trip meter. It can be reset back to zero at any time the driver chooses, whereas the odometer is not able to be reset by the driver. The trip odometer allows the driver to measure their current trip, which is handy for calculating gas mileage. I've used my trip meter for two things. I reset my trip meter after I fill up my gas tank all the way full, then when I fill up again I take the number the trip meter gives me and divide it by the amount of gas I was able to put in my gas tank. So I've taken my miles driven and divided by gallons used to get my recent MPG or Miles Per Gallon. The other use is for vehicles where the gas gauge no longer works or is inaccurate. Fill up a gas can with spare gasoline. Fill your vehicles gas tank to full. Reset the trip meter. Drive your vehicle until you completely run out of gas and the engine dies (do this in a place that would be safe to do so like a road out of town or small neighborhoods, not on main streets or in busy traffic). Look at your trip meter and take note of the number of miles you were able to get out of a tank of gasoline. That number is now E for Empty just like on your gas gauge. You can put the spare gas in the tank now so you can get to a gas fill up station. Now every time you get gas fill your tank all the way, and reset your trip meter. Plan to fill up about 100 miles before that Empty number so that you are safe, because if you are idling somewhere for an extended period of time, that is using gas and it is not being reflected in trip odometer reading. Two of my personal vehicles have had the gas gauge not work and I have used this method for the entire time I drove them, and have never run out of gas.
When an old style mechanical odometer would reach 99,999.9 miles, all of the numbered wheels would then "turn over" to zero. It would then read 00,000.0 as if the vehicle were brand new and had never been driven, despite the fact that it had been driven 100,000 miles! This could even happen several times and then would be referred to as having rolled over twice or three times or whatever had been the case. It was relatively common for older vehicles to have their odometers roll over. Newer vehicles that have hundred-thousands places do not roll over. A very long life of a vehicle would be 400,000 miles. Most won't make it that long, some may even make it longer, but to roll over these odometers a vehicle would have to have driven ONE MILLION miles, which is unheard of. It is a different case for semi trucks. They make it to ridiculously high mileage! I'm only talking about passenger cars and light trucks.
People in the know, that have mechanical skills, can sometimes change the odometer readings of vehicles. Changing an odometer with the intent of fooling a buyer is called "clocking" or "odometer tampering" and is VERY illegal. People still do it, though, because it is very lucrative. A person could buy a high mileage vehicle that looks nice for cheap, and roll back the odometer to something low, like 40,000 miles or less and sell it for a huge profit. I'll be writing a post about clocking soon, but for now I just wanted to make you aware of it.
I hope this helped you learn about how to read an odometer and a little about how they work. If you have any questions, please leave it in the comments below!
