Two clocks run on every tire
Tires wear out in two ways. The tread wears down as the tire rolls, and the rubber itself ages over time, whether or not the car is driven much. Either can be the reason to replace a tire, and a tire is due for replacement when it reaches the limit on whichever clock comes first.
Tread depth
Tread grooves have a job: they channel water out from under the tire and give the rubber edges to grip with. As the tread wears, the tire clears water less effectively, so wet braking distances tend to grow and the risk of aquaplaning increases. New passenger tires typically start with a tread depth somewhere in the range of 8 to 10 millimeters (roughly 10/32 to 12/32 of an inch), though this varies by model.
Legal minimums. In the United States, the usual legal minimum is 2/32 of an inch. In the UK and much of Europe, it is 1.6 millimeters. These are essentially the same depth. Rules vary by country and state, so check what applies where you live.
A legal minimum is not a recommended target. Many tire experts suggest replacing tires before reaching the minimum, especially if you drive in rain or snow, because performance in wet conditions drops noticeably as tread wears. A figure of around 4/32 of an inch (about 3 millimeters) is often mentioned as a point to start planning for replacement, and winter tires typically need more tread to work well in snow. The exact figures are general guidance, and the manufacturer’s advice is a good reference.
How to measure
Wear indicators. Most tires have small raised bars molded into the bottom of the main grooves. When the tread wears down level with these bars, the tire is at its legal minimum. Small marks on the sidewall (such as arrows or the letters TWI) show where the bars are. If the bars look flush with the surrounding tread, the tire needs replacing.
A tread depth gauge. A cheap gauge gives a direct reading. Insert the probe into a groove and press the base flat on the tread. Measure at several places across the tire, not only one, because wear may be uneven.
The coin test. In the U.S., a common check is to put a penny into a groove with the head of the president facing down. If you can see the top of the head, the tread is at or below about 2/32 of an inch. Other countries use similar tests with their own coins, but they are a rough check and not an accurate measure.
Check the inner, center and outer parts of each tire. If the wear is uneven, the pattern says something about the cause: pressure that is too low or high, an alignment problem, or worn suspension parts. The patterns are outlined in the guide to car tires and how they wear.
Age
Rubber hardens and cracks as it ages, influenced by heat, sunlight, ozone and time. An old tire can look fine and have deep tread, yet be more likely to develop cracks or separation inside the structure. The date is on the sidewall as part of the DOT code, and the last four digits give the week and year. As a hypothetical example, “2321” means the 23rd week of 2021. The article on sidewall markings shows where to find it.
There is no universal age limit. Many vehicle and tire manufacturers advise that tires be inspected regularly once they are around five years old, and a number of them recommend replacing tires at about six to ten years from the date of manufacture, even if they look serviceable. Check the owner’s manual for your car’s specific advice. This applies to the spare as well, which can age without ever touching the road.
Signs of age-related damage include:
- Fine cracks in the sidewall or between tread blocks
- Dry or brittle-looking rubber
- Bulges or blisters on the sidewall
- Vibration that appears without an obvious reason
A bulge or deep cracking is a reason to stop driving on the tire and have it inspected promptly.
Other reasons to replace
A tire may need replacing for reasons that have nothing to do with the tread or age:
- A puncture that cannot be repaired, as described in the article on slow punctures
- Sidewall cuts, bulges or exposed cords
- Damage from driving while flat or very low
- Severe uneven wear, which may be a sign of a mechanical problem to fix first
Replacing in sets
Ideally, tires on the same axle are replaced together, so tread depths match. Many vehicle makers advise that all four be matched on all-wheel-drive cars, because differences in tire diameter can strain the drivetrain. Check your manual. If you replace only two, the fresh pair is generally fitted at the rear, which many tire specialists recommend for stability, but follow the advice of a qualified technician.
A simple routine
- Look at your tires monthly when you check pressure.
- Measure tread depth every few months and before winter or long trips.
- Note the manufacture date of each tire, including the spare.
- Start planning a replacement when tread nears 4/32 of an inch or the tires near the age limit in your manual.
Tire wear and age limits vary by tire, use and climate, so when in doubt, ask a tire shop to inspect them.