#### Ulrich

##### Well-known member

Very cool ... but as usual I had my own thoughts to that ... the speedo shows a number calculated from the rotation of the transmission shaft .. some magic formula that includes the final drive and it must include the circumference of the rear tire .... after all we need to know how many inches (or centimeters) there are in one revolution ... and that is where I opened a can of worms (figuratively in my head) ... a new tire with the proper tire pressure has an 'X' inches (or centimeters) circumference ... but what about a worn tire with maybe a tiny bit of low pressure .. that would significantly alter the circumference of the tire ... and your speedo deviation will become larger ... just my 2¢ ... and no, this will of course not affect any GPS type reading .. completely different style of measurement ...## What is a speedometer?

The first thing will be to know for sure what a speedometer is. It is a device thatmeasures the rotation of the wheel and the transmission shaft.

This demolishes the first myth that the speedometer measures the speed at which you circulate.

To measure the rotation of the wheel, it makes use of a sensor that is nothing more than a magnet, which is in the gearbox.

It is the integrated electronic system that is in charge of estimating that rotation and therefore marking the speed at which it is traveling on the vehicle's speedometer. Hence there is some difference between the actual speed and the speed we read in the car.

## Why does the speedometer mark more speed?

It doesn't matter what the make of your car or the model, this is something that happens in all of them. The difference is that the margin of error in many cases is lower and in others a little higher. This is because there are some factors that determine the variation in the speedometer with the actual speed of the car.

To understand a little more about the reason for this difference,we have to go back to what the law says. According to the European vehicle type approval regulations, the speed of the speedometer can never be less than the actual speed. There has to be a certain margin of error but upwards.

More precisely, we mean thatthere must be an upward margin of error of 10% plus 4km/h.

With a practical case, it would be the following: a car going 100 km/h would have a margin of error of 14/km/h. If you see on the speedometer that it says 114 km/h, it will not be like that. The actual speed of the vehicle you are driving is 100 km/h.

## What is the law applied to the speedometer called?

This lawis called UN ECE Regulation 39and it applies when the vehicle is homologated within the European Union.

It makes it very clear that a lower speed can never be reflected on the speedometer.

There are countries such as the United Kingdom where other laws apply since it is not part of the European Union. There the upward law of 10% + 6.25 mph applies.

But it is not only in the European Union that this law exists, but for example in the United States it is also applied. Here vehicles are allowed to have an upward margin of error from 5 to 50 mph.

## What factor determines the speed measurement?

We indicate thatthe margin of error varies from one manufacturer to anotherand there are several factors that influence it.

One of the most affected isthe type of tire used in the car. A car with new tires will have a larger diameter since it does not have as much wear and every time it rotates, it will be able to have a greater rotation. So the speed in this case will be higher.

With the passage of time, when the diameter of the tire decreases due to the wear of the rubber itself, the drop in air, etc., the distance that will be traveled will be shorter. The car will go slower and there the speed measurement will also be slower, so there is a margin of error.

## Is GPS speed measurement accurate?

All GPS allow you to see how fast you are driving. This works both on GPS that are integrated into the vehicle and on external ones.

For their measurement, what they do is measure the distance that is traveled in a certain time using satellite tracking. With this tracking, they search for the position and calculate the route we have made and divide it by the time it took to make that route.

The result obtainedwill depend largely on the signal obtained by satellite tracking. It does not depend, as in the previous case, on the tires.

Another factor that affects and makes the measurement with the GPS not exact is thatat no time do they take into account changes in vertical direction.

GPS is much more accurate when we are going at high speed since there is more time in the same state. It is the only case in which the GPS measurement can be a little more accurate than with the speedometer.

They don't work as well within a city where we stop constantly. The errors here can be much more noticeable and have information that is not true or that cannot be verified.

It may interest you:The best apps for your car.

## So do I have to consider the speed of the speedometer?

Bearing this information in mind, it is best to drive at a speed slightly less than the speedometer indicates. Thus, you will know that you are really driving at the permitted speed.

Also, consider the external factors that we have discussed and that could affect your condition.

Always comply with the speed limits of each roadin order to comply with the law, take care of yourself and other drivers and avoid a fine from the DGT.

Now that you know that the speedometer does not indicate the actual speed, drive responsibly.

In any case .. I don't stare much at my Speedometer ... just when 'Johnny Law' is around .. and then I just hope I am under whatever the last sign said