Why your car’s speedometer does not show the real speed you are driving at

Automobile speedometers are usually not very accurate. In fact, all speedometers may show a higher speed than the actual speed. It happens in all cars, whatever the make or model, and it happens with both analog and digital dashboards.

The deviation between what the speedometer shows and the real speed at which the vehicle is driving, far from being a casual matter, is part of the law and is completely accepted by the industry, and we explain why.

The speedometer collects data from a sensor (magnet) located in the gearbox. This sensor is part of an electronic system which, by means of electrical impulses, sends a signal to the speedometer. The faster the car’s transmission rotates, the more electrical impulses are sent and the higher the speed indicated on the dashboard.

ABS sensors can also be used to calculate vehicle speed, but are less common. An older system, no longer in use, made use of wires with jagged ends that were used to obtain the vehicle’s speed.
Error Velocimeter 1

On the data obtained, each manufacturer includes a margin of error for voluntary excess. Why? Very simple. Imagine that an accident occurs when the car marked 118 km/h, but it was really driving at 129 km/h on a road with a speed limit of 120 km/h. It would be normal that the brand of the car involved in the accident would have some possibility of being sued. If in the case of the accident we are referring to a speeding fine, the example becomes even clearer. Although in this case the radars also work with margins of error.

The brands cover their backs in a legal way, assuming that their speedometers are inaccurate so that we drive at a lower speed than the one we perceive on the dashboard. In this way, car manufacturers avoid possible legal action against them.

At the legal level, it is forbidden for cars to display a lower speed than the one they are actually driving. The reason for this is that governments ensure that we actually drive slower, thus reducing the chances of an accident or the consequences of an accident. By driving at the maximum permitted speed, we will actually be driving at a slower pace.

The European regulation that deals with this problem is UN ECE Regulation 39. This text establishes that the permitted difference between the posted speed and the actual speed must be, at the most, equal to the actual speed multiplied by 0.1 plus 4 km/h.

For example, if the road limit is 120 km/h, a car actually traveling at that speed could clock 136 km/h, i.e. 12 km/h times 10% plus another 4 km/h maximum.

Japan and the United States have a stricter standard. This is the SAE J1226, which collects both excess and default a maximum deviation of 4%.

As a curiosity, it should be noted that despite its connection with both devices, the speed difference applied to the speedometer does not affect either the odometer or the rev counter (tachometer), since they are not induced any margin of error.

Leave a Comment