<P> Another early form of electronic speedometer relies upon the interaction between a precision watch mechanism and a mechanical pulsator driven by the car's wheel or transmission . The watch mechanism endeavors to push the speedometer pointer toward zero, while the vehicle - driven pulsator tries to push it toward infinity . The position of the speedometer pointer reflects the relative magnitudes of the outputs of the two mechanisms . </P> <P> Typical bicycle speedometers measure the time between each wheel revolution, and give a readout on a small, handlebar - mounted digital display . The sensor is mounted on the bike at a fixed location, pulsing when the spoke - mounted magnet passes by . In this way, it is analogous to an electronic car speedometer using pulses from an ABS sensor, but with a much cruder time / distance resolution - typically one pulse / display update per revolution, or as seldom as once every 2--3 seconds at low speed with a 26 - inch (2.07 m circumference, without tire) wheel . However, this is rarely a critical problem, and the system provides frequent updates at higher road speeds where the information is of more importance . The low pulse frequency also has little impact on measurement accuracy, as these digital devices can be programmed by wheel size, or additionally by wheel or tire circumference in order to make distance measurements more accurate and precise than a typical motor vehicle gauge . However these devices carry some minor disadvantage in requiring power from batteries that must be replaced every so often in the receiver (AND sensor, for wireless models), and, in wired models, the signal being carried by a thin cable that is much less robust than that used for brakes, gears, or cabled speedometers . </P> <P> Other, usually older bicycle speedometers are cable driven from one or other wheel, as in the motorcycle speedometers described above . These do not require battery power, but can be relatively bulky and heavy, and may be less accurate . The turning force at the wheel may be provided either from a gearing system at the hub (making use of the presence of e.g. a hub brake, cylinder gear or dynamo) as per a typical motorcycle, or with a friction wheel device that pushes against the outer edge of the rim (same position as rim brakes, but on the opposite edge of the fork) or the sidewall of the tyre itself . The former type are quite reliable and low maintenance but need a gauge and hub gearing properly matched to the rim and tyre size, whereas the latter require little or no calibration for a moderately accurate readout (with standard tyres, the "distance" covered in each wheel rotation by a friction wheel set against the rim should scale fairly linearly with wheel size, almost as if it were rolling along the ground itself) but are unsuitable for off - road use, and must be kept properly tensioned and clean of road dirt to avoid slipping or jamming . </P> <P> Most speedometers have tolerances of some ± 10%, mainly due to variations in tire diameter . Sources of error due to tire diameter variations are wear, temperature, pressure, vehicle load, and nominal tire size . Vehicle manufacturers usually calibrate speedometers to read high by an amount equal to the average error, to ensure that their speedometers never indicate a lower speed than the actual speed of the vehicle, to ensure they are not liable for drivers violating speed limits . </P>

Where is the speedometer located in a car