Odometers

Odometers are devices that measure the distance traveled by a vehicle. They are typically integrated into the vehicle's instrument cluster and display the total distance in miles or kilometers. Odometers are distinct from speedometers, which measure current speed; the odometer records cumulative travel distance, often used to assess maintenance intervals, estimate resale value, and verify usage for leasing or warranty purposes. In some cases, a separate trip odometer can be reset to track a specific journey.

Mechanically, odometers have relied on gear trains linked to the rotating wheels. Each wheel revolution advances a geared mechanism that in turn increments a series of digits. Modern electronic odometers rely on wheel speed sensors, magnetic encoders, or GPS data processed by a vehicle's microcontroller. Digital systems may store distance in kilometers or miles and can be integrated with onboard telematics or trip meters that can be reset. Tire size affects accuracy; larger tires rotate fewer times per mile and can cause odometers to under-read, while smaller tires over-read. Some systems compensate for tire size in software or through calibration.

Historically, the concept dates to ancient engineering; Hero of Alexandria described an odometer that counted wheel revolutions to measure distance. The machine-age and the rise of motor vehicles led to practical odometers in the late 19th and early 20th centuries; digital odometers became common in the late 20th century. Odometer readings are subject to regulation: in many jurisdictions, sellers must provide an accurate odometer disclosure at transfer of ownership to prevent odometer fraud, which is illegal and punishable by law. Regular checks and calibration help ensure the displayed distance reflects actual travel.