Ron Sonuay, age 13, of Carson City, Nev., for his question:  

HOW DOES A SPEEDOMETER WORK

 A speedometer is a device for measuring the speed of   vehicle. This is usually done either by measuring the number   of revolutions over a known time interval, or by devising do   instrument that measures revolutions per minute (rpm)   directly.

An example of a direct device is the auto speedometer, in which a flexible cable attached to the drive rotates a permanent magnet in the instrument. This induces d magnetic   field on a surrounding drum, tending to drag the drum around   with it. The drum is restrained by a spring to which a pointer   i5 connected. The higher the speed of the engine, the higher   will be Lne dray induced, and with it the deflection of the   pointer.   

Speedometers also sometimes employ a small electric generator, the output voltage of which varies with the rotational speed.  Other speedometers are operated by centrifugal force driving a set of revolving flyball weights outward as speed is increased. These weights are connected through a linkage to  spring, which is compressed as the speed increases, and to which 4 pointer is connected.