Tread Depth, Age and Sidewall Damage: When Tires Need Replacing

Three checks cover almost every reason a tire needs replacing: how much tread is left, how old the rubber is, and whether the structure has been damaged. A tire can pass one and fail another. A set with deep tread can still be too old to trust, and a nearly new tire can be ruined by one hard pothole, so it is worth running all three every time you look.

Check one: tread depth

Tread depth is measured in 32nds of an inch in the US. A new passenger tire usually starts somewhere around 10/32 to 11/32 inch, and most states set the legal minimum at 2/32 inch. Legal and safe are not the same thing, though. As the grooves get shallower, they move less water out from under the tire, so stopping distances on wet roads grow and hydroplaning starts at lower speeds well before the legal limit.

Every passenger tire sold in the US has tread wear indicators molded into it. These are small raised bars running across the bottom of the main grooves, set at 2/32 inch. Many tires mark their locations with a small triangle or the letters TWI on the shoulder. When the surrounding tread wears down flush with a bar, the tire is at the legal minimum and should be replaced.

Two coins give a quick reading before that point:

• Penny test. Push a penny into a groove with Lincoln's head pointing down. If you can see the top of his head, the tread is at or below 2/32 inch.
• Quarter test. Do the same with a quarter. If the top of Washington's head is visible, the tread is at or below about 4/32 inch.

Many tire safety groups and testers, Consumer Reports among them, suggest replacing tires at around 4/32 inch if you drive in rain or snow, because traction falls off noticeably below that depth. Measure in the inner, center and outer grooves, and on more than one spot around the tire. If one edge or the center is much lower than the rest, the tire is worn unevenly, which usually points to an inflation, alignment or suspension problem that a shop should look at before new tires go on, or the new set will wear the same way.

Dedicated winter tires lose much of their snow grip well before the legal limit, so many makers suggest retiring them from winter duty at around 6/32 inch.

Check two: age

Rubber hardens as it ages, even on a spare or a car that sits most of the week, and an old tire can lose grip or separate internally while the tread still looks deep.

The manufacturing date is on the sidewall as part of the DOT code: the letters DOT followed by a string of characters, ending in four digits. The first two digits are the week and the last two are the year, so 3421 means the 34th week of 2021. The full code may only be printed on one side, so check the inner sidewall if the outer one shows a shorter code.

There is no single federal age limit for passenger tires, and advice differs by maker. Many tire and vehicle manufacturers recommend a professional inspection every year once a tire passes 5 or 6 years old, and replacement somewhere around 6 to 10 years from the manufacture date regardless of tread. Your vehicle's owner's manual may give its own figure. Look for fine cracks in the sidewall or between the tread blocks; heavy cracking on an older tire is a sign to replace it sooner. Check the date on a used tire before buying it, and on a new one at installation, since tires can sit in a warehouse for months.

Check three: damage

Run a hand and an eye around the whole tire, including the inner sidewall, with the wheels turned to full lock so you can see it.

• Bulges or bubbles on the sidewall mean the internal cords have broken, often after a pothole or curb strike. The tire can fail suddenly. Replace it before driving any distance on it.
• Cuts and gouges deep enough to show cord or fabric mean the same.
• Punctures from nails and screws are often repairable if they sit in the main tread area and are no bigger than 1/4 inch across.

A proper puncture repair, under the guidelines published by the US Tire Manufacturers Association, means taking the tire off the wheel, inspecting the inside, and fitting a combined plug and patch from the inside. A plug pushed in from the outside alone is not considered a complete repair. Damage to the sidewall or the shoulder, where the tread meets the sidewall, cannot be repaired at all, because those areas flex too much to hold a patch. A tire that was driven any distance while flat may also be beyond repair, since the inner liner can be crushed even if the outside looks fine.

If a tire loses air repeatedly with no visible nail, ask a shop to check the valve stem and the bead where the tire seals against the wheel before assuming the tire itself is finished.