Speedometer

From Free net encyclopedia

A speedometer measures the speed of a vehicle.

Traditional automotive speedometers are driven by a flexible, sleeved cable that is rotated by a set of small gears in the tail shaft of a transmission. The speedometer itself is two rotaing, barrel-shaped magnets. One magnet is connected the sleeved cable, and the other is connected to the speedometer needle. These magnets are calibrated such that a given revolution speed of the flexible cable corresponds to a specific speed indication on the speedometer. This calibration must take into account several factors, including ratios of the tailshaft gears that drive the flexible cable, the final drive ratio in the differential, and the diameter of the driven tires. The speedometer mechanism often also drives an odometer.

Modern speedometers are electronic. A rotation sensor delivers a series of electronic pulses whose frequency directly correspond to vehicular speed. A computer converts the pulses to a speed and displays this speed on a digital display or an electronically-controlled, analog-style needle, the latter of which is more common. Pulse counts may also be used to increment the odometer.

As of 1997, federal standards allowed a maximum 5% error on speedometer readings (per "Auto Tutor", American Automobile Association of California magazine, Oct. 17, 1997). Aftermarket modifications, such as different tire and wheel sizes or different differential gearing, can cause speedometer inaccuracy.

Template:Commons Speedometers for other craft have specific names and use other means of sensing speed. For a boat, this is a pit log. For an aircraft, this is an airspeed indicator.

[edit]