How accurate is your speedometer?

It’s a good idea to check the accuracy of your car’s speedometer every so often to ensure you’re not driving faster or slower than you think. Speedometers rely on sensors located inside your vehicle’s transmission to measure how fast the wheels spin. Therefore, improperly inflated or worn tires can cause your speedometer to malfunction.

How to test your speedometer
To test your speedometer you’ll need a stopwatch, a friend, and a stretch of road with mile markers along with it. Start the test by driving at a steady speed and ask your companion to start the stopwatch as you pass a highway marker.

Once you’ve passed 10 markers (10 miles), have your friend pause the stopwatch and take note of how long it took you to travel the distance.

To determine your speed, divide 60 (the number of minutes in an hour) by the number of minutes it took you to drive the 10 miles, and then multiply this figure by the distance you traveled. For example, your equation might be:

60 ÷ 10 minutes x 10 miles = 60 miles per hour

You can also use a smartphone app that uses GPS to calculate speed, such as Google Maps, to get a better idea of how fast you’re going.

Don’t panic if your speedometer isn’t right on. If it’s off by two or three miles per hour, just keep that discrepancy in mind when you’re driving so you don’t inadvertently speed. In fact, in the United States, it’s acceptable for your speedometer to be off by up to four percent. However, if your speedometer is off by more than five miles per hour, it’s a good idea to bring it to a mechanic to have it inspected and recalibrated.