Clocking and Legality
A form of fraud is to tamper with the reading on an odometer and presenting the incorrect mileage to a prospective buyer; this is often referred to as clocking. This is done to make a car appear to have been driven less than it really has been, and thus increase its apparent market value. Many new cars sold today use digital odometers that store the mileage in the vehicle's engine control module making it difficult (but not impossible) to manipulate the mileage electronically. With mechanical odometers, the speedometer can be removed from the car dash board and the digits wound back, or the drive cable can be disconnected and connected to another odometer/speedometer pair while on the road. Older vehicles can be driven in reverse to subtract mileage, a property that provides the premise for a classic scene late in the comedy film Ferris Bueller's Day Off, but modern odometers add mileage driven in reverse to the total as if driven forward, thereby accurately reflecting the true total wear and tear on the vehicle.
The resale value of a vehicle is often strongly influenced by the number of miles or kilometres a passenger vehicle has on the odometer, yet odometers are inherently insecure because they are under the control of their owners. Many jurisdictions have chosen to enact laws which penalize people who are found to commit odometer fraud. In the US (and many other countries), vehicle mechanics are also required to keep records of the odometer any time a vehicle is serviced. Companies such as Carfax then use this data to help potential car buyers detect whether odometer rollback has occurred.
Read more about this topic: Odometer