Speedometer

A speedometer is a device that measures the instantaneous speed of a land vehicle.

Now universally fitted to motor vehicles, they started to be available as options in the 1900s, and as standard equipment from about 1910 onwards.

Speedometers for other vehicles 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.

The speedometer was invented by the Croatian Josip Belušić in 1888, and was originally called a velocimeter.

Operation

Eddy current

The eddy-current speedometer has been used for over a century and is still in widespread use. Until the 1980s and the appearance of electronic speedometers it was the only type commonly used.

Originally patented by a German, Otto Schulze on 7 October 1902, it uses a rotating flexible cable usually driven by gearing linked to the tail shaft (output) of the vehicle's transmission. The early Volkswagen Beetle and many motorcycles, however, use a cable driven from a front wheel.

A small permanent magnet affixed to the rotating cable interacts with a small aluminum cup (called a speedcup) attached to the shaft of the pointer on the analogue instrument. As the magnet rotates near the cup, the changing magnetic field produces eddy currents in the cup, which themselves produce another magnetic field. The effect is that the magnet 'drags' the cup -- and thus the speedometer pointer -- in the direction of its rotation with no mechanical connection between them.

The pointer shaft is held toward zero by a fine spring. The torque on the cup increases with the speed of rotation of the magnet (which, recall, is driven by the car's transmission.) Thus an increase in the speed of the car will twist the cup and speedometer pointer against the spring. When the torque due to the eddy currents in the cup equals that provided by the spring on the pointer shaft, the pointer will remain motionless and pointing to the appropriate number on the speedometer's dial.

The return spring is calibrated such that a given revolution speed of the cable corresponds to a specific speed indication on the speedometer. This calibration must take into account several factors, including ratios of the tail shaft 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 plus a small switch that sends pulses to the vehicle's engine computer.

Electronic

Many modern speedometers are electronic. A rotation sensor, usually mounted on the rear of the transmission, delivers a series of electronic pulses whose frequency corresponds to the rotational speed of the drive shaft. The sensor is typically a toothed metal disk positioned between a coil and a magnetic field sensor. As the disk turns, the teeth pass between the two, each time producing a pulse in the sensor as they affect the strength of the magnetic field it is measuring.

A computer converts the pulses to a speed and displays this speed on an electronically-controlled, analog-style needle or a digital display, the latter of which is more common nowadays. Pulse counts may also be used to increment the odometer.

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.

Error

Speedometers are not totally accurate, and most speedometers have tolerances of some 10% plus or minus due to wear on tires as it occurs. Modern speedometers are said to be accurate within 5% but as this is legislated accuracy, this may not be entirely correct. This can make it difficult to accurately stay on the speed limits imposed; most countries allow for this known variance when using RADAR to measure speed, although levels of some 3 km/h or 3% are also used in areas of tough enforcement. This causes many arguments due to motorists complaining that they were not doing the speed as reported. Revenue[3] is being increasingly blamed for these stricter measures. There are strict United Nations standards in place but it seems not being enforced leaving this matter in limbo for many countries. Excessive speedometer error after manufacture can come from several causes but most commonly is due to nonstandard tire diameter, in which case the

Nearly all tires now have their size shown as "T/A_W" on the side of the tire , and the tire's

GPS

GPS devices may indicate the true speed of travel on the user interface. Unlike instrumental speedometers which provide a continuous reading, most GPS speed readouts have a one-second update interval.

The reading is based on reception of data from the satellites in orbit, and is therefore independent of the car's transmission components. Discrepancies between the two readings may be caused by instrument error (on the vehicle), or by changing directly influential factors, such as tire sizes.