Low tire tread depth can be a safety issue. When the road is wet, low tread depth can cause the tire to hydroplane, or skim across the surface of the water, which can make you lose control. You also have less traction in the snow, and the tire is more susceptible to a blowout. Although the ideal way to check tread wear is with a tread-depth gauge, you can get by with what’s known as the “penny test” or just using your fingers.
Let Your Fingers Do the Checking
New tires typically start with a tread depth of 10/32 to 11/32 of an inch, and they’re generally considered legally “worn out” when the tread is down to 2/32, though some consider 4/32 as a safer minimum.
Nearly all tires have built-in wear bars across the tread at four equally spaced places around the circumference of the tire. These wear bars are 2/32 of an inch tall and typically are in a straight line across the tread, but sometimes they are just a little offset from one another.
If you run your finger along any of the longitudinal tire grooves, you’ll likely feel the wear bar as a little hump. If you find it, any tread that stands equal to the height of the wear bar means the tread depth is down to 2/32 — or possibly below (since both could already be worn down) — and the tire should be replaced. Any amount of tread standing above that can be used to estimate its depth. If the tread feels to be about twice the height of the wear bar, you’ve got roughly 4/32 tread remaining.
The Penny Test
The penny (or similar quarter) test is also commonly used to help judge tread depth.
If you hold a penny so that Abraham Lincoln’s head is facing down and insert it into a groove, the top of Lincoln’s head is about 2/32 of an inch from the coin’s edge. So if you can see Lincoln’s whole head, your tread depth is down to 2/32 or perhaps less, and you’re due for new tires.
If you hold a quarter so that George Washington’s head is facing down and insert it into a groove, the top of Washington’s head is about 4/32 of an inch from the coin’s edge. If you can see Washington’s whole head, you’re at a point where you’d probably want to start thinking about replacing the tires.
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Check Three Grooves, Not Just One
When you’re checking tread depth, it’s best to check it in three different grooves: the one closest to you, the one in the middle and the groove farthest away. Any difference in those depths typically indicate improper tire inflation or a wheel alignment issue.
Less tread depth in the center groove than in the outer grooves indicates the tire has been running overinflated. That’s because higher pressure causes the center of the tread to bulge out, carry more weight and wear faster. If there’s more tread in the center — which can be likely — the tire has generally been running underinflated.
If the tread depths vary in steps from the near groove to the far groove (such as 5/32 in the nearest groove, 4/32 in the middle groove and 3/32 in the farthest groove), it indicates a suspension alignment issue with camber. Negative camber, which could cause the measurements noted above, means the top of the tire tilts inward; positive camber means the top of the tire tilts outward. A little bit of difference isn’t a big deal and you may be able to even out tire wear by having them rotated (assuming it’s only the front or rear tire on each side that’s “off”). Note that on some cars, the camber can’t be adjusted, so if it’s way off, it indicates a worn or bent suspension component.
While You’re at It …
Something else you can check using your hand is for feathering and cupping. Note that this will get your hand dirty unless you’re wearing thin gloves; thick winter gloves or work gloves probably won’t provide enough feel.
To check for feathering, run your hand across the tread near side to far and back again. If you feel as though there’s an edge catching your hand going in one direction but not the other, that could mean there’s feathering and, possibly, a problem with toe adjustment — either toe-in or toe-out. Toe-in is a steering alignment issue that means the leading edge of both front tires point slightly inward toward the center of the car rather than being parallel; toe-out is the opposite. Either can usually be corrected with a fairly simple adjustment.
To check for cupping, or low spots on the tire, flatten your hand and run it along the top of the tread (right to left and back) for about a quarter of the circumference of the tire. This indicates either that the tire is out of balance (which you may feel when driving as a shimmy in the steering wheel) or that there’s an alignment or suspension problem.
If you look at the sidewall of a tire and notice tiny cracks in it, the tire has dry rot, which usually happens when the tire is quite old, though climate conditions can hasten the deterioration. Regardless of your tread depth, the tire should be replaced, as dry rot can cause the tire to blow out.