Functions of the Odometer

An odometer tracks the distance traveled by an automobile, truck or other vehicle, and is featured in the dashboard. Motorcycles have an odometer fixed at the base of the handlebars, into a dash, or atop the gas tank. An odometer can be mechanical or digital.
A mechanical odometer basically consists of a series of cogs featuring numbers on each edge. The cogs turns in accord with wheel rotation via the odometer cable and drive mechanism. The mechanical odometer is hidden from view by a windowed casing that reveals only a single row of numbers, which displays current mileage. This can be viewed on the speedometer face. Depending on the age of the vehicle, a mechanical odometer might have a maximum count of 99,999 miles, at which point it rolls over to start recounting from 00000 miles.

The modern electronic or digital odometer tracks mileage using a computer chip. The readout is digitally displayed and the mileage is stored in the main engine control module. Manufacturers hoped the electronic odometer might prevent odometer fraud, but this hasn’t been the case.
Resale value of a vehicle is based in large part on mileage. All else being equal, the fewer miles a vehicle has, the higher the resale value. Odometer fraud, or clocking involves manually setting the odometer back to falsely deflate mileage and inflate the value of the car. 
Clocking a mechanical odometer is a fairly easy task. Dishonest private parties and unethical retailers can remove the odometer, manually turn the cogs to display the desired mileage, then reinstall the device. Innocent buyers can pay hundreds or even thousands of dollars more than a car’s actual worth, often discovering odometer fraud too late.
The automobile industry expected the digital or electronic odometer to significantly reduce odometer fraud. However, tools needed to legally calibrate digital odometers for legitimate purposes are readily available online and off. These tools are known to be used to set back digital odometers. Experts claim this is actually easier than turning back a mechanical odometer.
To protect yourself against odometer fraud compare the mileage on the vehicle with the title. Each time the vehicle changes hands, the mileage must be listed on the title. If the vehicle is newer with 20,000 miles or less on the odometer it should still have the original tires. For older cars with low miles, check wear on the gas, brake and clutch pedals. Ask for maintenance records and/or smog check records and note the mileage.  
If considering a car with a mechanical odometer, the numbers should line up evenly. If they are crooked or visibly move when you rap the dash or the face of the odometer, experts advise walking away.
Finally, utilize a company like CarFax or AutoCheck to receive a report on the vehicle using the vehicle identification number (VIN). These reports not only verify odometer readings, but will list accidents, flooding, or other reported mishaps that have befallen the vehicle.