Odometer

 
An odometer (often known colloquially as a mileometer or milometer) 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' (an older name for this device is hodometer).

Recently, exercise enthusiasts have observed that an advanced Gobal Positioning Satellite (GPS) receiver (GPSr) with an odometer mode serves as a very accurate pedometer for outdoor activities. While not truly counting steps (no pendulum is involved) an advanced GPSr odometer can reveal the accurate distance traveled to within 1/100th of a mile (depending on the model, perhaps 1/1000th of a mile). 1/1000th of a mile is approximately the distance of a single pace or 2 steps.

The modern definition of the international mile traces back to the Roman military method of keeping track of how far a soldier had traveled on foot. The Latin "mille passus" is literally "a thousand paces" where 1 pace = 2 steps. The international mile (5,280 US feet) is somewhat longer than the original Roman mile (4,854 US feet). As with the mile, the definition of "foot" has changed many times!

A GPSr with odometer mode is also an excellent and inexpensive means to verify proper operation of both the speedometer and odometer mounted in a vehicle.

Synopsis

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; in the early days of the automotive industry this was adequate. With continuous improvements, modern vehicles now survive to travel several hundred thousands of miles/kilometers. 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 (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. Modern odometers now add mileage driven in reverse to the total as if driven forward, to accurately reflect the true total wear and tear on the vehicle.

Most modern cars also include a trip meter, also referred to as a trip odometer. Unlike the odometer, a trip meter is designed to be reset at any desired point in a journey, making it possible to record the distance traveled in any particular journey or part of a journey. It was traditionally a purely mechanical device but, in most modern vehicles, it is now electronic. Luxury vehicles often have multiple trip meters. Most trip meters will show a maximum value of 999.9. There are practical uses for this:

Record the distance traveled on each tank of fuel, making it very easy to accurately track the energy efficiency of the vehicle.

When following driving directions, reset it to zero at each turn to be sure when one has arrived at the next turn.

Law

The resale value of a vehicle is often strongly influenced by the number of miles or kilometers 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.