Can Car Speedometers Be Wrong?

How accurate are speedometers? Dan Edmunds, an automotive engineer and the director of vehicle testing at Edmunds.com, says that speedometers cannot have an error of more than 5 percent (typically expressed as plus/minus 2.5 percent relative to the actual speed) according to federal law.

Are speedometers in cars accurate?

"The accuracy of the speedometer in most vehicles, including Volkswagens, is generally within a few percentage points of actual speed," Tetzlaff says. ... If the diameter of the wheels changes – and it does change, depending on tire size, pressure and wear – the accuracy of the speedometer gets thrown off.

How do I know if my speedometer is accurate?

If you think your speedometer is inaccurate, you can use a stopwatch to check how accurate it is. Start the watch as you pass a mile marker while on the highway, and then stop it once you pass the next marker. The second hand on your stopwatch will be your speed.