When to Replace Tires: Tread Depth, Age and Warning Signs

How much tread is too little?

In the US, tread depth is measured in 32nds of an inch. New tires typically start at about 10/32 or 11/32 inch, and some truck, SUV and winter tires start deeper.

The floor is 2/32 inch. That is the legal minimum in most states, but rules vary, so check your state’s vehicle code or inspection rules. New York’s inspection, for example, requires at least 2/32 inch in the two adjacent major grooves with the most wear. Passenger and light truck tires also have tread wear indicators: narrow raised bars molded across the bottom of the grooves at 2/32 inch. Once the tread is flush with those bars anywhere on the tire, the tire is worn out.

Legal is not the same as good. Grooves carry water away from the contact patch, and shallow grooves carry less of it. In AAA’s 2018 testing, with tires buffed down to 4/32 inch, a midsize sedan needed 42% more distance to stop from 60 mph on wet pavement than on new tires, and a full-size pickup needed 44% more. At the point where the sedan stopped on new tires, it was still going about 39 mph on the worn set. AAA recommends that drivers start shopping for new tires once tread falls below 4/32 inch.

Tire makers also warn that a tire at the legal minimum may not hold off hydroplaning or keep its grip in rain and slush. If you count on your tires in winter weather, don’t run them down to the limit.

Two coin tests and a gauge

You don’t need special tools for a rough reading.

• Penny test (2/32 inch). Put a penny into a main groove with Lincoln’s head pointing down. If you can see all of his head, the tread is at or below 2/32 inch and the tire should be replaced.
• Quarter test (4/32 inch). Repeat with a quarter, Washington’s head down. If the tread covers part of his head, you have at least 4/32 inch left. If his whole head shows, you are below 4/32 inch, where wet-road stopping gets noticeably worse.
• Tread depth gauge. A simple probe-style gauge from an auto parts store gives you an actual number in 32nds of an inch.

Measure in several places around each tire, across the inner, center and outer grooves. If any spot reaches 2/32 inch, replace the tire, even if the rest of the tread is deeper.

What uneven wear is telling you

Even wear across the tread is normal. Uneven wear usually points to a problem with inflation, alignment or suspension. Fix the cause before new tires go on, or the new set may wear the same way.

• Center worn more than the edges: usually overinflation.
• Both outer edges worn: usually underinflation.
• One edge worn: usually an alignment problem, often camber.
• Feathered ribs or saw-tooth tread blocks: often alignment settings, such as toe, that are out of spec.
• Cupping or scalloping (a wavy, dipped pattern): often wheel imbalance or worn shocks or struts.

Regular rotation helps even out wear. If your vehicle or tire maker doesn’t give a schedule, the U.S. Tire Manufacturers Association (USTMA) suggests rotating every 5,000 to 8,000 miles, or at any sign of uneven wear. Monthly pressure checks address one of the most common causes of uneven wear.

Damage that ends a tire’s life early

Plenty of tires come off with tread left on them. Look for:

• Bulges or blisters in the sidewall or tread. USTMA lists them among the conditions that should take a tire out of service.
• Cuts, cracks or snags that expose cord or fabric. Fine surface cracking from sun and ozone, called weather checking, is worth watching. Severe cracking, or any crack that reaches the cords, means the tire should come off.
• Vibration, pulling or new noise. These can come from uneven wear, internal damage or a balance problem. Have a qualified tire shop take a look.
• A hard hit. After a big pothole or curb strike, damage inside the tire may show little or nothing on the outside at first. Have it inspected.

Which punctures can be repaired?

Under USTMA repair guidelines, a passenger or light truck tire can be repaired only in the tread area, and only if the injury is no larger than 1/4 inch (6 mm) across. Damage that reaches into the shoulder or sidewall is not repairable. A proper repair means taking the tire off the wheel, inspecting the inside, filling the hole and sealing the inner liner with a patch. A plug alone or a patch alone is not a proper repair, and tire makers may add their own limits, for example for certain speed ratings or run-flat tires.

One more catch: USTMA warns that a tire driven even a short distance while underinflated may be damaged inside beyond repair, even when it looks fine from the outside.

How old is too old?

Rubber changes with time, even on a tire that is rarely driven. Spares and tires on vehicles that sit for long stretches can age out before they wear out.

To find a tire’s age, look for the DOT code on the sidewall. On tires made since 2000, the last four digits give the week and year of manufacture, so “3520” means the 35th week of 2020. A three-digit date code means the tire was made before 2000, far past the 10-year limit some tire makers set.

There is no single industry-wide age limit. USTMA says it knows of no scientific or technical data that sets a specific minimum or maximum service life, because heat, storage, load and use vary so much from tire to tire. Some tire makers do set their own guidance. One major maker recommends a yearly inspection by a tire professional once a tire has been in service for five years, and replacement 10 years after the date of manufacture regardless of tread, spare included. Another recommends taking its tires out of service after 10 years, whatever the tread depth. Your vehicle maker may list its own limit in the owner’s manual, so follow the recommendations that apply to your tires and vehicle.

Replacing two tires, one, or all four

USTMA says replacing all four tires at the same time is preferred. When that isn’t possible:

• Two tires: put the new pair on the rear axle. Deeper tread clears water better, and more grip at the rear helps keep the back of the vehicle from sliding out on wet roads.
• One tire: USTMA suggests pairing it with the tire that has the deepest remaining tread and putting both on the rear axle.
• Matching: USTMA recommends the same size, speed rating and construction on all four, unless your vehicle maker specifies different front and rear sizes. Never choose a tire with less load capacity than the original size at the placard pressure.

All-wheel-drive and four-wheel-drive vehicles need extra care. Some vehicle makers advise against replacing fewer than four tires, and USTMA warns that on these vehicles even small differences in outside diameter between tires may damage the drivetrain. Check the owner’s manual for any size or tread-depth matching rules before you buy.

When you are ready to shop, start with the size on your door-jamb placard, then compare all-season tires, highway/touring tires, passenger car tires or SUV/CUV tires in that size.