As cars have gotten more and more complicated, the technology responsible for creating an accurate odometer reading has remained fairly simple. In fact, there are really only two common methods for keeping track of vehicle mileage.
The driving force for the older manual (or mechanical) odometer style begins at the transmission. A small gear is incorporated into the transmission solely for the purpose of advancing the odometer. One end of a part often referred to as the “speedometer drive cable” is secured to this gear. The other end of the cable attaches to the back section of the instrument cluster.
The internal transmission gear turns any time the car is moving. This turning motion is transferred by the drive cable to another small set of gears attached to the changeable digits inside the car. The counting process begins at the right side of the set of numbers, and continues until an appropriate amount of distance forces the next digit to the left to roll over. This process continues until all digits have reached their maximum value, at which point they all roll back over to zero.
The digits on a manually driven odometer will not normally line up to form a perfect row. This is a natural result of the counting process, and a set of perfectly matching digits may have been altered.
Unlike the older mechanical gauges, digital odometers rely on electronics to create an accurate mileage count. A special gear is still used to change the dashboard counter, but instead of a cable, a magnetic sensor monitors the amount of gear turns in the transmission. This signal is transferred by wire to the vehicle computer. The computer then translates the information into a mileage figure.
If a gauge cluster fails, the appropriate odometer adjustments should be performed at the same time the replacement is installed. With most modern clusters, this occurs at the dealership via a computer scan tool, or is set by the gauge manufacturer before it is sent to the parts department.
This extra computer control means that modern systems are much more carefully guarded than the older cable style. Long gone are the days when a dishonest used car salesman could easily alter vehicle mileage to squeeze extra money out of unsuspecting shoppers. This, along with other technological advances, has made car buying a bit safer for the average driver.