Along with proper inflation, knowing how to measure your tread depth is an essential skill for good tire maintenance. Besides just knowing when the tires need to be replaced, there are advantages to keeping an eye on tread depths, especially for things like:
- Knowing when snow tires are about to lose a great deal of performance. (6/32” or less)
- Being able to track if one part of the tread is wearing faster than the others, to catch irregular wear issues before they can damage the tire.
There are basically three ways to measure tread depth; the right way, the quick-and-dirty way and the quicker-and-dirtier way. All three work to tell you if your tires need to be trashed for new ones, but only the right way will tell you anything else.
The right way is to use a gauge. Tread depth gauges are cheap and easy to stow 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 millimeters.
I, of course, recommend using the gauge. If you do use one, you might want to check tread depth every month or so, and check multiple spots along the tread. By doing this you can catch irregular wear, like that caused by alignment issues, long before it becomes an irrevocable problem.
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. If the head is covered at all, the tire is above 2/32”.
Finally, there's the quicker-and-dirtier way. 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 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.
I know it may not seem like much, but getting a little bit picky about tread depth can definitely pay off in the long run in longer tire life and a better ride when the tires wear evenly.