Speedometer

A speedometer is a specialized gauge designed to measure and display a vehicle's instantaneous speed. This means it shows how fast a vehicle is moving at a specific moment in time. While they are now found in almost every motor vehicle, they became common options in the early 20th century. By approximately 1910, they were included as standard equipment on many automobiles. Other types of transportation use similar devices to monitor speed through different methods. For example, boats utilize a device known as a pit log to measure movement. Similarly, aircraft use an airspeed indicator to track how fast they are traveling through the air.

Mechanical speedometers have several different ways of working. Some early models used the governor principle, where a rotating weight moved against a spring as speed increased. Another type was the chronometric speedometer, which measured distance traveled over a precise interval of time using an escapement. Many common mechanical models use a rotating flexible cable linked to the vehicle's transmission. In this setup, a small permanent magnet on the cable interacts with an aluminum cup called a speedcup. This cup is attached to the pointer shaft on an analogue instrument. As the magnet rotates near the cup, it creates eddy currents within the metal. These currents produce a new magnetic field that exerts torque on the cup. This torque "drags" the pointer in the direction of the rotation without any physical connection.

The movement of an eddy current speedometer is a delicate balance of forces. A fine torsion spring holds the pointer toward the zero position. As the vehicle's speed increases, the magnet rotates faster, which increases the torque on the cup. The pointer moves until the torque from the eddy currents is balanced by the opposing torque of the spring. Because the torque is proportional to the vehicle's speed, the angle of the pointer also shows the speed. This allows manufacturers to use equally spaced markers on a dial to represent speed gaps. However, these mechanical systems have a limitation regarding direction. They cannot show speed when a vehicle is in reverse gear. In that scenario, the cup turns in the opposite direction, pushing the needle against its mechanical stop pin.

Modern vehicles have largely transitioned to electronic speedometers. In these designs, a rotation sensor is mounted in the transmission to monitor movement. This sensor might use magnets on an output shaft or a toothed metal disk. As the parts turn, the teeth or magnets pass a sensor and create a series of electronic pulses. The frequency of these pulses corresponds to the rotational speed of the driveshaft. Some manufacturers even use pulses from ABS wheel sensors via a CAN Bus system. A computer then converts these pulses into a speed reading for the driver. This can be shown on a digital display or an electronically controlled analogue needle.

Unlike mechanical versions, electronic speedometers can easily display speed while moving in reverse.

History shows that several inventors contributed to the development of these tools. Charles Babbage is credited with creating an early type of speedometer used for locomotives. In 1888, the Croat inventor Josip Belušić patented an electric version called a velocimeter. He presented his invention, which used a magnet and electricity, at the 1889 Exposition Universelle in Paris. Later, German inventor Otto Schultze patented a version in 1902 that also utilized eddy currents. These historical advancements paved the way for the highly accurate digital systems used in modern transportation.

Speedometers can also be "virtual" or specific to smaller vehicles like bicycles. Virtual speedometers use satellite radio navigation systems, such as GPS, to approximate speed based on distance and time. These can be less accurate over short distances and may be affected by weather or terrain. Bicycle speedometers often use a sensor that pulses when a magnet on a wheel spoke passes by. Older bicycle models might use a cable driven from a wheel or a friction wheel against the tire rim. While these are useful, they can be bulky or require battery power. Accuracy in all these systems is vital for safe and legal travel.

Despite their precision, speedometers have a tolerance of about ±10%. This error often stems from variations in tire diameter caused by wear, pressure, or temperature. If a tire's diameter changes, the distance covered in one rotation changes too. For example, replacing a standard tire with a smaller one can cause the speedometer to read higher than the actual speed. To prevent drivers from accidentally speeding, manufacturers often calibrate speedometers to read slightly high. In many countries, international agreements like the UNECE Regulation 39 set strict standards. These rules ensure that the indicated speed is never lower than the actual speed.