When to Replace Tires: Tread Depth, Age and Damage Checks

Check tread depth first

Tread is the rubber that meets the road, and the grooves in it are what push water out of the contact patch. As a tire wears, those grooves get shallower and wet-road grip falls, which increases the risk of hydroplaning. That is why any replacement decision should start with a tread depth check (tread depth on new tires).

The federal safety standards define tread wear indicators as the projections in the principal grooves designed to give a visual indication of the degree of wear. When the tread is worn even with the top of those bars, the tire has about 1/16 inch (2/32 inch) of depth left, which is the standard minimum (tread wear indicators).

The penny test is the quick way to check the same threshold. Insert a penny into the tread groove with Lincoln's head pointing down; if the top of his head is covered by tread, you have more than the minimum, and if you can see the top of his head, the tire is at or below the replacement point. A tread depth gauge is more precise, and it is worth measuring in the center and both shoulders of the tire, because the shallowest spot decides.

Inspect for cracks, bulges and cuts

Tread depth is only part of the story. A tire can have full tread and still be unsafe if the structure is damaged. The federal standards define cracking as any parting within the tread, sidewall or inner liner that reaches cord material, and they treat separations of the tread, ply or sidewall as structural failures that make the tire unfit for further service.

A bulge in the sidewall is a red flag. It usually means the internal belts or cords have broken, often from an impact with a pothole or curb, and the tire cannot be repaired. Cuts, punctures and embedded objects deserve the same attention. Run your hand over the tread and both sidewalls, and look closely at the area where the tire meets the rim and in the tread grooves.

If you can see cord or fabric through a cut or crack, the tire has lost the structure that holds it together, and it should be replaced immediately. A thorough visual inspection catches most of these problems before they become failures (how to inspect tires).

Age matters even with tread left

Rubber ages even when it is not driven. Heat, sunlight and ozone slowly harden the compound, and after enough years a tire can develop cracks or lose grip even if the tread has barely worn. That is why a low-mileage spare or a lightly used set can still need replacement.

Every tire sold in the United States carries a DOT identification code molded into the sidewall, and the last four characters of that code give the week and year of manufacture (tire DOT date code). For example, a code ending in 2619 means the tire was made in the 26th week of 2019. That date is your starting point for the age question.

Because rubber ages, tire makers publish guidance on the age at which a tire should be retired regardless of tread. Follow the guidance for the specific tire you own. When in doubt, an older tire showing fine cracks between the tread blocks or on the sidewall has reached the end of its service life, even if a penny test says the tread is fine.

Uneven wear is a clue

The tread on a healthy tire wears evenly across the contact patch. When one area wears faster than the rest, the tire is telling you something about pressure, alignment or suspension. Sharply worn shoulders with a full center usually point to under-inflation, a worn center with full shoulders points to over-inflation, and cupping or a sawtooth pattern often comes from worn suspension parts or poor alignment.

Uneven wear matters for two reasons. It shortens the life of the current tire, and it will destroy a replacement the same way if the cause is not fixed. So before you buy new tires, correct the pressure, alignment or suspension problem (causes of tread wear). Rotating the tires on schedule keeps wear even over the life of the set (tire rotation patterns).

Pressure and the TPMS

Proper inflation is the cheapest way to keep tires in service. The Department of Energy notes that under-inflated tires lower fuel economy, and properly inflated tires are safer and last longer. Use the pressure on the vehicle's door placard, not the maximum pressure molded on the tire sidewall (tire pressure basics).

If your vehicle has a tire pressure monitoring system, it has to warn you when a tire falls 25 percent below the recommended cold inflation pressure. That warning is a maintenance signal, not a replacement signal, but it is worth treating seriously. A tire that keeps losing pressure has a slow leak, which can be a puncture, a bead leak or a damaged valve, and the tire needs to be inspected to find the cause (TPMS systems).

Repair or replace?

Not every damaged tire needs to be scrapped. A small puncture in the tread area, away from the shoulder, can often be repaired safely when the casing is sound. But cracks, bulges, sidewall damage, and punctures that have damaged the internal structure are not repairable.

The federal standard for retreaded passenger tires shows which conditions rule a casing out: exposed bead wire or cord fabric, removed belts or plies, and any processing that exposes cord material. If a tire is not acceptable as a casing for retreading, it is not acceptable as a tire on your vehicle. When you are not sure, a tire technician can tell you whether a repair is possible or the tire needs replacement.

When it is time to replace, put a matched set on the vehicle whenever you can (sets of four). If you are replacing only two, match the new pair to the remaining tires in size, type and tread characteristics. Then choose a tire that fits how you drive (replacement guidelines).

What to pick for your use

Questions

How do I do the penny test?

Insert a penny into a tread groove with Lincoln's head pointing down. If the top of his head is covered by tread, the tire has more than the minimum depth. If you can see the top of his head, the tread is at or below about 1/16 inch, and the tire should be replaced.

What does the DOT number on my tire tell me?

The DOT number is the tire's identification code. The last four characters indicate the week and year of manufacture. For example, 2619 means the tire was made in the 26th week of 2019. Use this to determine how old the tire is.

Can a tire with a bulge be repaired?

No. A bulge in the sidewall or tread usually means the internal belts or cords are broken, which compromises the structure of the tire. It cannot be safely repaired and should be replaced.

At what tread depth should I replace my tires?

The federal minimum is 1/16 inch, which is what tread wear indicators are molded to show. When the tread wears down to that point, the tire is legally worn out and should be replaced. Many tire makers recommend replacing earlier for better wet-road performance.

Do tires expire even if they have plenty of tread?

Yes. Rubber compounds age and harden over time, and heat and sunlight accelerate the process. Tire makers publish age limits, often around six to ten years, after which the tire should be replaced regardless of remaining tread depth.

What is the difference between a crack and a cut in a tire?

A crack is a parting in the rubber that can develop from age or ozone exposure, while a cut is caused by external objects. Both are dangerous if they reach the cord material. If you can see cords through the crack or cut, the tire needs to be replaced.