the car dashboard 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 concept which provides the premise for a classic scene 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 total distance shown 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 or inspected. Companies such as Carfax then use these data to help potential car buyers detect whether odometer rollback has occurred. Prevalence Research by Irish vehicle check specialist Cartell found that 20% of vehicles imported to Ireland from Great Britain and Northern Ireland had had their mileometers altered to show a lower mileage. Accuracy Most odometers work by counting wheel rotations and assume that the distance traveled is the number of wheel rotations times the tire circumference, which is a standard tire diameter times pi (3.1416). If nonstandard or severely worn or underinflated tires are used then this will cause some error in the odometer. The formula is (actual distance traveled) = ( (final odometer reading) – (initial odometer reading) ) * (actual tire diameter) / (standard tire diameter). It is common for odometers to be off by several percent. Odometer errors are typically proportional to speedometer errors. See also Mechanical counter Opisometer Roadometer (odometer) Speedometer Surveyor's wheel Tachograph Tachometer Taximeter Taffrail log References Sources External links American inventions Ancient Greek technology Chinese inventions English inventions Greek inventions Length, distance, or range measuring devices Vehicle parts Vehicle technology