If you’ve ever noticed those radar signs that show how fast you’re driving seem to be a bit off, you’re not the only one. This is because most vehicle’s speedometers are inaccurate and usually read faster than the vehicle is actually traveling in order to account for errors.
Regulation concerning speedometers varies by country, but European and United States regulations are the common standards manufacturers follow. The variance in European nations is 10% of the true speed +4 KPH and must never display a speed less than the true speed. In the U.S. the speedometer at 50 MPH must be within ± 5 MPH of the true speed.
Many manufacturers sell to both markets so they want to comply with both regulations. To do this this many speedometers read within 0 to +5 MPH error since this range complies with both regulations and allows about 10% error at highway speeds.
Since even a slight change in tire pressure or a new set of tires can throw the speedometer off slightly many read a few miles per hour over so they’re in the middle ground and any minor error will still keep them within regulation and out of trouble.
An example would be as shown in the info graphic. At a true speed of 50 MPH your speedometer will likely display anything from 50 MPH to 55 MPH. If the speed limit is 55 MPH you may think you’re going the speed limit while actually going 5 MPH slower.