THE MODERN SPEEDOMETER evolved from the odometer, which goes back to Roman times. To determine how far they had traveled, Roman legions rolled a wheel as they marched and multiplied its circumference by the number of revolutions. By the sixteenth century the odometer had been combined with a clock to yield a kind of speedometer. Through the stagecoach and railroad ages, calculating average speed over an interval was usually good enough. No one felt much need for an instant speed reading.

But when motorists started kicking up dust at the turn of the century, speed limits inevitably followed, and drivers needed to know how fast they were going. The earliest automotive “speed meters” arrived around 1901. Motorists could choose among four basic types: centrifugal, hydraulic, pneumatic, and magnetic.

The centrifugal type worked like the flyweight governors on stationary engines, with two balls swinging outward from a rotating shaft (usually geared to a front wheel) on pivoted arms. By reading the height of the balls, the driver could tell how fast he was going.

The hydraulic type had a column of mercury inside a small vertical tube that also housed a hydraulic screw. A cable from the car’s front wheel turned the screw and raised a float on top of the mercury. The faster the wheel was turning, the higher the float rode. The pneumatic type measured the air pressure in a tube mounted in the direction of the car’s motion. These became common on aircraft but not in automobiles because fluctuations in ambient air pressure proved too much of a problem at low speeds.

Finally there was the magnetic speed meter. While more complicated than the others, it was also more reliable and accurate, so it won out. In this type of speedometer a permanent magnet spins inside an aluminum cup, with its speed of rotation proportional to the car’s speed. The spinning magnet induces “eddy currents” in the aluminum cup, which create a magnetic field of their own. As a result, the cup tries to rotate along with the permanent magnet. Its rotation is opposed by a weak spring.

One of the first successful magnetic speed meters was developed by A. P. Warner of Beloit, Wisconsin, who founded the Warner Instrument Company in 1904. He bought out several other speedometer makers, and by 1925 his Stewart-Warner Company was supplying 90 percent of all U.S. carmakers. Meanwhile, Henry Ford kept many small manufacturers in business by omitting the speedometer from his Model T after 1915, thus ensuring a thriving after-market.

At first, many speedometers had tiny readouts that were viewed through an inch-square glass window. Numbers were silk-screened around the outside of the horizontal aluminum cup, and as the cup turned, the appropriate numeral showed through. Even with a magnifying lens, the little numbers were hard to read, especially at night. By the 1930s most carmakers had replaced the window readout with the more prominent dial version. To create this, manufacturers simply turned the aluminum cup ninety degrees and attached a needle to its pivot center.

In the early 1980s automakers began introducing electronic speedometers and odometers. While mechanical speedometers were and still are highly accurate (though usually set to read fast, to reduce speeding tickets), electronic types are even more so. They are also more tamperproof. During the fuel-crunch 1970s and 1980s, when the nationwide speed limit was lowered to 55 mph, car manufacturers set the top figure on their speedometers at 80. Today, with speed limits up to 70 mph on most interstates and no limit at all in some places, most speedometers max out at well over the century mark.