Mechanical odometers usually appear as a row of wheels with the edge of the wheels towards the person viewing it. There are digits written on the edge 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.
In the U.S., odometers on older cars could only indicate miles up to 99,999 miles. At 100,000 miles, the odometer would restart from zero. This is known as odometer rollover. Newer cars usually have odometers that can indicate up to 999,999 miles.
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 used less than it actually has been, to get a higher price for the car.
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History
An odometer for meauring 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.
Chariots with wheels of 4 feet diameter turns exactly 400 times in one Roman mile. For each revolution, a pin on the axle engage a 400 tooth cogwheel, thus making one complete revolution per mile. This engages another gear with holes along the circumference, where pebbles (calculus) are located, that drop one by one into a box. The number of miles travelled is given simply by counting the number of pebbles. Whether this instrument was actually built is disputed. Leonardo da Vinci tried to build it accoring to the description, but failed. Andre Sleeswyk made a working model, however.
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