Odometer, or odograph, is a device that measures the distance travelled by an automobile based on the perimeter of the wheel as the wheel rotates. The odometer gives us only the measure but doesn’t show the direction of velocity. The odometer measures the scalar quantity – the speed of the car. The device may be electronic or mechanical or a combination of the two.
Most odometers work by counting wheel rotations and assume that the distance travelled is the number of wheel rotations times the circumference, which is the standard tire diameter times pi (3.146). Some errors will be caused in the odometer, if nonstandard or severely worn or underinflated tires are used. It is common for odometers to be off by several percent.