Speedometer: how it works and why it gives errors in speed measurement

We all know the speedometer, that element of the car that tells us how fast we are going at any given moment. But although it is one of those very common elements we use without barely noticing, it is a great unknown to most users. It is even unknown that it has an error in measuring speed. This error has several explanations, which we will discuss below, but first we want to show you the two types of speedometers that exist and how they work.

Types of speedometer

There are two types of speedometers: an older one and a more modern one fitted to the most current models. Both are based on the same concept, although the way of achieving it differs. To know the speed at which a vehicle is travelling according to the speedometer, one must know the diameter of the wheel and the number of revolutions it makes in a given time.

To find the first, one must calculate the size of the rim plus twice the tyre profile. Normally the diameter is known, because the manufacturer knows which type of tyres the car is fitted with. All that remains, then, is to understand the two types of speedometers that exist and how they work.

Mechanical speedometer

Mechanical speedometers were fitted to cars a few years ago. They are simpler than current ones and measure speed through the rotation of a cable connected to the secondary shaft of the gearbox. This rotational speed is proportional to that of the wheels according to the transmission ratio of the differential.

At the other end of the cable there is a magnet. As it rotates, it generates an electric current that moves the speedometer needle across a graduated scale. To determine that scale and make it as accurate as possible, all the forces involved in that movement must be taken into account, such as the torsional resistance of the spring, which must have been calibrated according to the wheel diameter, the intensity of the induced current, the differential ratio and other variables. In this way the display of the vehicle’s speed can be provided.

Electronic speedometer

The electronic speedometer is more modern and its design is more practical, simpler and more reliable. A rotation sensor is placed on the secondary shaft of the transmission, which sends a variable voltage current to the control unit; the unit interprets it and converts it into the speed signal that is then shown on the instrument cluster, in analogue or digital format.

The rotation sensor is also called a Hall-type sensor, which consists of two magnets that rotate at the same time. Between those two magnets is the sensor, which generates an electric current. The rotation of the magnets regroups the electrons generated and so the reading is obtained, which is then converted into data in the control unit.

The electronic speedometer can also determine the vehicle’s speed using the ABS tone wheel. This system may introduce some additional error in the measurement, since on a bend one wheel turns slightly less than the other.

Speedometer error

Speedometers produce an error in the speed measurement. It is not a very large error, but it can reach a little over 10% of the real speed. And here’s a surprise: this error is regulated by law, and it is the European Union that establishes that the speed shown on the speedometer must be higher than the real speed. This difference between the real speed and the speed shown on the speedometer is set at 10% plus 4 km/h.

That is, a car travelling at 120 km/h according to the speedometer would actually be moving at a lower speed. This difference creates a psychological effect on the driver, since even though they believe they are driving at the road’s maximum speed, that speed is never actually reached.

In Japan or the United States, a deviation of 4% between the real and indicated speed is permitted, both above and below. Other factors that influence speedometer reading errors may be mechanism wear, the use of wheels with a different diameter from that for which the system was calibrated, and tyre pressure. The latter can make the car roll more or less, affecting both speed and distance travelled.