An odometer keeps track of the distance your car has been driven. This information can help drivers remember when to get their oil changed or perform tune-ups, and it also plays a role in the resale value of a vehicle. Most older vehicles relied on mechanical systems of gears to count the miles, but newer cars typically have digital odometers that measure the distance traveled electronically.
Mechanical odometers are very basic, yet entirely accurate, systems. They rely on an extremely high gear ratio to track the distance a vehicle has traveled. Gear ratios measure how many rotations a smaller gear has to make for the larger one to complete one cycle. For example, a two-to-one ratio means that the smaller gear circles two times for every single rotation of the larger gear. In the case of mechanical odometers, the larger gear typically represents a mile.
Ferris Bueller's Day Off may have led viewers to think it is impossible, but running a car with a mechanical odometer in reverse actually can turn back the clock on older models. Since this is a real possibility, drivers in the market for older cars with mechanical mile-counters should be wary of tampering, which is a fairly common automotive scam.
Check the odometer to ensure the numbers are lined up straight, and watch it during test drives to see if it sticks at all. Look for fingerprints on the underside of the plastic cover that protects the dash instruments. Vehicle service stickers, maintenance records and the car's title can also provide mileage information that could be used to uncover fraud. If the car's title date is recent, it could mean that a new one was created to cover up the mileage tampering.
Electronic odometers, unlike their predecessors, are tamper-proof. Most electronic mile-tracking systems in automobiles rely on a toothed wheel on the transmission output and a magnetic sensor that tracks the pulses created as the teeth pass by. This information is translated into mileage and tracked by the vehicle's engine control unit (ECU). The ECU stores the overall mileage, so even if someone does find a way to change the digital display readout, a diagnostic computer can extract the correct mileage from the ECU.