Car speedometer: how does it work and how much does it really mark?

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The speedometer measures the speed of the vehicle (from the Greek tachys, speed, hence other terms such as tachograph, tachycardia and many others) and is therefore one of the most important elements of vehicle instruments. This is why the law has made it mandatory for all vehicles exceeding the speed of 25 km / h. In fact, the obligation was staggered according to European standards: the last categories to have the speedometer by law were 2 and 3-wheel vehicles (1 July 2001) and mopeds, from 1 January 2002.


This tool, which is mandatory by law, allows the driver to know at what speed a vehicle is traveling. We remind you that speed is expressed as the ratio between a distance and the time taken to travel it: for road vehicles, the unit of measurement kilometers per hour (or miles per hour) is generally used, abbreviated to km / h. The speedometer is a tool that has gone through a deep technological evolution, since the first major series was built in 1923 by the Otto Schulze Autometer and its basic design has not changed significantly for 60 years. The operating principle is to count the revolutions of the transmission at the output of the gearbox or directly the revolutions of the wheels: knowing the number of revolutions and the circumference of the wheels, it is possible to extrapolate the vehicle speed from these quantities.

Speedometers have been magnetic for a long time: a flexible cord connected to the transmission rotated a magnet facing a metal disc integral with both the speedometer needle and a spiral spring. The metal disc would tend to turn ‘following’ the magnet with a force proportional to the rotation speed of the magnet itself – and therefore to the speed of the car – but the spring contrasts it and therefore the disc (and the index finger attached to it) stops in a position that is balanced between its tendency to rotate and the strength of the spring. The higher the speed, the more the force of the disc increases and the more the spring is compressed, with the index moving more and more on the dial.

The development of electronics has retired lines, magnets and springs, replaced by pulse generators – connected to the transmission or wheels – and counters of these pulses: the higher the speed, the more frequent the pulses will be. In this case, the speedometer under the driver’s eyes is an instrument that counts the impulses in the unit of time (their number in one second is proportional to the wheel revolutions) and calculates the corresponding speed to show it to the driver, driving stepper motors able to rotate by small fractions of a degree instead of continuously. This data can be displayed in analog form, with a needle that moves on a dial (possibly visualized in virtual on a display, photo below), digitally with numerical digits or in both ways.



THE electronic speedometers they could be a lot precise, but they are not because their indication must be greater than the actual speed. The European authorities have in fact prescribed that the speed indicated by the speedometer must not never be lower than the actual speed so that it must not be possible to exceed the speed limits due to a speedometer error. However, there is an upper limit: the indicated speed cannot exceed 110% of the actual speed plus 4 km / h. If we go to 100 km / hour the indicated value cannot exceed 114 km / h while at 50 the speedometer cannot mark more than 59 km / h.

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If a mechanical speedometer no longer works or the needle moves irregularly, the first suspect is the flexible cord that causes the magnet to die. If this is not damaged then the fault may be in the instrument itself. Electronic instruments should last longer because they don’t have the sweet spot of the flexible lanyard. In the event of a malfunction it is though complicated to get your hands on but having an electronic diagnosis done (or done, if you are able) will be able to understand the origin of the fault.


The article is in Italian

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