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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 hodós, meaning 'path' or 'way', and métron, 'measure'.

Contents 1 Synopsis 2 History 3 Law 4 Sport 5 See also 6 References 7 Notes 8 External links

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 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 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.

They can be sometimes found in the top of the speedometer.

Possibly the first evidence for the use of an odometer can be found in the works of the Roman authors Pliny (NH 6. 61-62) and Strabo (11.8.9). Both authors list the distances of routes traveled by Alexander the Great (r. 336-323 BC) as measured by his bematists Diognetus and Baeton. However, the high precision of the bematists's measurements rather indicates the use of a mechanical device. For example, the section between the cities Hecatompylos and Alexandria Areion, which later became a part of the silk road, was given by Alexander's bematists as 529 English miles long, that is with a deviation of 0.4% from the actual distance (531 English miles). From the nine surviving bematists' measurements in Pliny's Naturalis Historia eight show a deviation of less than 5% from the actual distance. Three of them even less than 1%. Since these minor discrepancies can be adequately explained by slight changes in the tracks of roads during the last 2300 years, the overall accuracy of the measurements implies that the bematists already must have used a sophisticated device for measuring distances, although there is no direct mentioning of such a device.

An odometer for measuring distance was first 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 later invented in ancient China by Zhang Heng (78 – 139 AD), perhaps diffused along the silk road, though no evidence indicates this.

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 modern systems, where a separate gear controls 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.

Benjamin Franklin invented a simple odometer when he was going on trips in carriages. He wanted to know how far he was going, and the speed he was travelling.^[1]

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 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.

• Hubometer
• Speedometer
• Tachograph
• Tachometer
• Taximeter

• Sleeswyk, André Wegener "Vitruvius' Odometer", Scientific American 245.4 (October, 1981), pp. 188-200
• Sleeswyk, Andre W. "Vitruvius' Waywiser", Archives internationales d'histoire des sciences Vol. 29 (1979), pp. 11-22.
• Donald W. Engels: Alexander the Great and the Logistics of the Macedonian Army, Los Angeles 1978, p.157f.

• Vitruvius' description
• History of the Odometer
• Odometry in Determination of the Position of an Autonomous Mobile Vehicle
• DLand Pro A digital odometer's manufacturer
• Relojería LEO A digital odometer's manufacturer
• "How Odometers Work" by Karim Nice