The modern odometer is very much like a calculator. Typically, a series of magnets placed on a rotating shaft or axle generates a series of pulses to a computer module as the vehicle travels.

The computer then translates the "number of pulses received" into"distance travelled" according to a formula programmed by the manufacturer of the instrument and/or the vehicle. The accuracy of the vehicle's odometer system is often simply a function of how precisely the manufacturer programs the computer.

If, in fact, ten pulses from the sensors should mean the car has traveled ten feet, tampering may be as simple as programming the computer to believe that ten pulses means the vehicle has traveled twelve feet.

While older, mechanically driven odometers often had errors of +/-4%, modern odometers should not.