You might not be speeding even if you think you are

Idea

If you have a personal navigation device (aka GPS) that shows the current speed you may have noticed that it rather consistently shows speed that is lower than the speed on the speed meter. Which measurement is more trustworthy?

The measurement of the navigation device should be more precise than the measurement provided by car. The navigation device calculates the distance between the positions where the device was in the past second (approximately) and divides it by the elapsed time. The result is the average speed in the previous time period.

Car’s measurement of the speed is based on revolutions of the wheels per time period. Number of revolutions times perimeter of the tire yields the distance the car made in the given time period. The variable part of the formula is the perimeter of the tire. There are several sizes of tires and therefore different perimeters. Furthermore, other factors (such as air pressure in the tires) have influence on the perimeter of the tire. Car manufacturers tend to calibrate the speed meters so that they show the real speed for the “worst” conditions (largest possible tire perimeter). For smaller tires, the speed on the speed meter is therefore less than the actual speed.

For most of the cars, the difference between the two speed measurements is about 3%. I saw an instance where it was almost 10% (perhaps combination of small tires, low air pressure in the tires and inaccurately calibrated speed meter).

Thus you can drive many cars slightly faster than you thought without going over the speed limit. You can find the difference speed meter speed and actual speed using a personal navigation device. If you consider buying one, make sure you select one that shows the current speed (not all of them do).

Always make sure you drive safely and obey the traffic rules.

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