A modern non-digital odometer
A Smiths speedometer from the 1920s showing odometer and trip meter
A modern digital odometer
Rolling from 99,999 to 100,000 miles
An odometer is a device used for indicating distance traveled by an automobile or other vehicle. It may be electronic or mechanical. The word derives from the Greek words hodōs, meaning "path" or "way", and mētron, "measure".
Mechanical odometers usually appear as a row of wheels with an edge of each wheel exposed to the driver. There are digits written on the edges of these wheels. A mask obscures these wheels from view, except for one row of digits which can be seen through a window in the mask.
On older cars, odometers could only indicate up to a value of 99,999. At 100,000, the odometer would restart from zero. This is known as odometer rollover. Newer cars usually have odometers that can indicate up to a value of 999,999.
A common form of fraud is to tamper with the reading on an odometer. 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 theoretically possible 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 left disconnected while on the road (this also disables the speedometer).
An odometer for measuring distance is described by Vitruvius around 27 and 23 BC. The actual invention may
have been by Archimedes during the First Punic War. Hero of Alexandria describes a similar device in chapter 34 of his Dioptra
. The device was also invented in ancient China by Zhang Heng (78 – 139).
The odometer of Vitruvius was based on chariot wheels of 4 feet (1.2 m) diameter turning 400 times in one Roman mile (about 1400 m). For each revolution a pin on the axle engaged a 400 tooth cogwheel thus turning it one complete revolution per mile. This engaged another gear with holes along the circumference, where pebbles (calculus) were located, that were to drop one by one into a box. The distance travelled would thus be given simply by counting the number of pebbles. Whether this instrument was ever built at the time is disputed. Leonardo da Vinci tried to build it according to the description but failed. Later, Ben Franklin invented his own version. In modern times, however, Andre Sleeswyk was able to make a working model using gears similar to the Antikythera mechanism as opposed to the traditional cogwheel.
The odometer as used in most modern systems, where separate gears control each digit, was invented by William Clayton with help from Orson Pratt. Clayton, a Mormon Pioneer, developed the odometer (dubbed the "roadometer") to keep track of wheel revolutions on the pioneer wagons. The odometer had at least two gears, including one which turned every quarter-mile and one which turned every ten miles.
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, vehicle maintenance workers 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.
Odometers feature in some sports, both amateur and professional. Odometers designed for cycling help cyclists to determine distance cycled and often other information. (See cyclocomputer) Professional rally cars are usually equipped with a purpose-built odometer with an adjustable factor. This factor determines the number of wheel rotations in, say, one kilometre or one mile. Amateur rally cars are often also equipped with purpose-built adjustable odometers for regularity rallying.