Are car speedometers accurate? In short no. But this doesn’t mean that you shouldn’t trust the speed. Dan Edmunds who is the Director of vehicle testing at Edmunds.com said that the speedometer cannot have an error of more than 5 percent. In other words if your speedometer says that you are traveling at a speed of 60mph then your speedometer cannot read more than 61.5mph or less than 58.5mph.
Depending on your tire size the reading can be different. If your tires are a different size from stock or if your tread is wore down it can affect the accuracy of your speedometer. Edmund says,
The tires size steps are small enough that the diameter can easily be kept within a percent or two, which allows the same gearing and speedometer calibration factors to be carried over because everything still fits within the speedometer tolerance,
If the tires are a larger diameter than stock they can make your speedometer read less than the actual speed that you are traveling.
Want to test the accuracy of your speedometer. Head to the nearest highway. Grab a friend with a stopwatch and have then start the watch as you’re passing a mile marker. Maintain a speed of 60 miles per hour. If you have cruise control it can provide a better controlled experiment. As you pass the next mile marker stop the watch. The small or second hand on the watch will be able to tell you how fast you were going in a mile. It’s that easy!
So are car speedometers accurate? Not exactly but they are close. Check your speedometer accuracy and you might save yourself from a speeding ticket!