How to Measure the Tread Depth of Your Tires

An example of good tire tread
The Tire Zoo / Flickr / Creative Commons

Along with proper inflation, knowing how to measure your tires' tread depth, and with which tool, is an essential skill for good vehicle maintenance. There are a number of factors that contribute to tire aging, but older is never better. Bald tires are a serious hazard on the road. According to a recent study, nearly a quarter of the auto accidents that take place in the U.S. each year are due to insufficient tire tread. Drive on bald tires during a day that's too hot, cold, or rainy—or go a little too fast or brake a little too quickly—and you are in danger of spinning out or blowing out.

There are other advantages to keeping an eye on tire wear as well. Tread depth can indicate when snow tires are about to lose their performance (6/32" or less), and it will help you catch irregular wear before damage sets in to the tire or the wheel. If you see irregular wear, it's definitely an indication that you might need to have your tires balanced.

There are basically three ways to measure tread depth: the quick-and-dirty way, the quicker-and-dirtier way, and the right way. All three will tell you if your tires need to be trashed for new ones, but only the right way will tell you anything else.

Penny Test

The quick-and-dirty way to tell if your tires are legally bald is to use the penny test, which is probably almost as old as Abraham Lincoln himself. Put a penny into the gap between tread blocks with Abe's head facing down. If you can see all of Lincoln's head, your tread depth is below 2/32” and the tire is legally bald. Replace immediately. If the president's head is covered at all, the tire is above 2/32”.

Wear Bars

The quicker-and-dirtier way to test tire tread depth is to stick your finger in between the tread blocks. You should feel some little platforms in between the blocks, but sunk down well below the tread surface. The platforms are located at 2/32”, so when the tread surface becomes even with these “wear bars,” the tire needs to be replaced. If you can still feel some “meat” on the tread, all is not yet lost.

Gauges

If you want to test your tire tread depth the right way, you'll need to use a gauge. Available at just about any auto and tire store, tread depth gauges are inexpensive and they stow easily in a glove compartment. Most are a simple graduated mechanical probe, but there are fancy-schmancy digital models available as well. Stick the probe into the valley of the tread, press the shoulders of the probe flat against the tread bloke, and read off the result. All gauges should measure in both 32nds of an inch and in millimeters.

You should use a tire depth gauge to check tread depth every month or so, and check multiple spots as well. By doing this you can catch irregular wear, like that caused by alignment issues, long before it becomes an irrevocable problem.