It is rare that I drive without assistance of some kind, either with Android Auto or with the phone placed in a holder. And helping me, generally applications like Waze or Google Maps (generally the first, since I like it more, whether integrated into Google’s infotainment system or as a navigation app per se). So the normal thing is that the vehicle control panel coexists with that little help to reach my destination better. Over the years I have always observed something that has caught my attention: My car’s speedometer reads a different speed than Google Maps and Waze and the faster I go, the bigger the difference..

Although there may be a difference between some cars and others, the trend is the same since it obeys a regulation that aims to ensure that you stick to the speed of the road, thus avoiding putting your integrity and fines at risk.

The speed of Google Maps and Waze vs that of the speedometer

In fact, the crux of the issue is, on the one hand, how to obtain that speed and, on the other hand, in the UN ECE Regulation 39. This regulation refers to the approval of the speed indicator device and the odometer and applies to Europe. , but similar rules apply elsewhere. In any case, it requires that ‘The indicated speed (editor’s note: from the car speed indicator device) may not be less than the actual speed of the vehicle‘.

Or what is the same, the speedometer must show a speed that either corresponds to the real speed or is somewhat higher. But never inferior. Logically, Like any other instrument, it has a margin of in this case the manufacturers are clear that this margin of failure must go upwards.

In fact, in the regulations there appears the equation that relates the speed indicated (V1) by the speedometer and the actual speed (V2):

0 ≤ (V1 – V2) ≤ 0.1 V2 + 4 km/h

As an example, if your actual speed is 100 km/h, your speedometer may show a speed of up to 14 km/h more. Thus, if you are going on a secondary road with a good shoulder and according to the speedometer you see that you exceed the speed established by law (100 km/h), common sense will tell you reduce your speed to stick to the characteristics of the road and also, don’t get fined.

If the previous equation is difficult for you to understand, in words it is simpler: in Europe the maximum margin of error of the car speedometer with respect to the real speed is 10% + 4 km/h.

But if you have Android Auto running or simply with a navigation application open, you will be surprised to see a different speed. What speed is good? At what speed should I pay attention? Depends.

Navigation applications show a speed that they have determined thanks to the relationship between the variation of our geolocation (data taken with GPS) per unit of time. Although there are also errors and inaccuracies in GPS (its approximate precision ranges from 1 to 3%), In general the speed shown will be closer to the real one.

For information, having a speed that is practically the real one is fine and if both one and the other are below the speed stipulated by law for that particular road, it means there is still room to step on the accelerator. But If we are on the edge of what the law establishes, be careful because an oversight It can mean having less room for maneuverability by going faster than you should and also penalties (here there is also another melon to open: the margin of error of the radars).

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