Your speedometer doesn't show your car's real speed. Do you want to know why?

Knowing the speed you are travelling at all times is very important. Above all, to keep track of the speed limits at every moment and avoid exceeding them. But is your car’s speedometer completely reliable? Well, although it may surprise you, it does not show exactly the speed at which you are travelling. Don’t worry, though: normally it reads somewhat more than your actual speed, so you won’t have to worry about going slightly faster than permitted.

In fact, speedometers normally register a speed slightly higher than the one you are actually travelling at. Legally, they may read up to 10% + 4 km/h below the real speed. This means that if you are on a road where the limit is 120 km/h and your speedometer shows that speed, you are actually travelling at a speed between 108 and 120 km/h.

If you are travelling at 70 km/h on the speedometer, the real speed is between 60 and 70 km/h, and if you are travelling at 100 km/h, the real speed will fall within the range of 86 to 100 km/h.

There is no way to know what your real speed is from the speedometer, although if it shows 120 km/h, we can be sure that the real speed is lower than that figure. One way to be safer and know that you will avoid a fine, even if at some point you slightly exceed those 120 km/h.

Speed is measured by a sensor located in the gearbox, which measures the rotation of the wheel and the drive shaft. This measurement can therefore have some deviation, which can reach around 2 km/h depending on tyre pressure or the vehicle’s load.

How to find your car’s real speed?

The speedometer is not completely accurate, as we have already seen. But there are ways to know your car’s real speed. One of those ways is to use a GPS. These devices, which can be external or integrated into the car, show the real speed or, at least, a speed much closer to reality than the speedometer. With this system, you can see the difference between the speed shown by the GPS and the one displayed on the instrument panel.