How to Check Tire Wear and Pressure (and When to Replace)

Your tires are the only part of your car that actually touches the road. Every bit of acceleration, braking, and steering happens through four palm-sized contact patches — one under each tire. Yet tires are one of the most overlooked items in routine car care. Worn tread, wrong pressure, or undetected cracking can quietly rob you of stopping power and control. This guide walks you through exactly how to check tread depth, find and set the correct pressure, read uneven wear patterns, and know when it is time to replace — no special tools beyond a coin and a gauge required.

Reading Tread Depth — Wear Indicators, Coin Test, and Legal Limits

Every tire sold for road use has built-in wear indicators (also called slip signs in Japan). These are small rubber bars moulded into the grooves at a depth of exactly 1.6 mm. When the tread surface wears down to the same height as these bars, the tire has reached its legal and safety limit and must be replaced.

To find the wear indicators, look at the side of the tire for small triangular arrows or the letters TWI (Tread Wear Indicator) moulded into the sidewall. Follow the arrow into the groove and you will see the rubber bar sitting at the bottom. If the tread is flush with that bar, the tire is done.

The coin test gives you a quick at-a-glance check between indicator inspections. In Japan, a 100-yen coin works well: insert it groove-first into the tread. If you can see the full characters on the coin (the outer numeric ring), the tread is likely below 3 mm and replacement should be planned soon. A US quarter coin gives a similar reading at about 4/32 inch (3.2 mm) — if you can see the top of the head, it is time to shop.

• New tire: typically 7–9 mm of tread depth
• Replace soon: tread around 3 mm — performance in wet weather declines noticeably
• Legal limit / replace immediately: 1.6 mm — wear indicators flush with tread
• Winter tires: replace when tread reaches 4 mm — winter grip drops sharply before the standard legal limit

Check tread depth at multiple spots across the tire width (center and both shoulders) and at several points around the circumference, because uneven wear can cause one spot to be legal while another has already passed the limit.

Checking and Setting Correct Tire Pressure

Tire pressure is measured in kPa (kilopascals) in most countries, or psi (pounds per square inch) in the US. Your car has a recommended inflation pressure specific to your vehicle — and it is not printed on the tire itself. The number on the tire sidewall is the maximum pressure the tire can hold, not the target. You need the vehicle placard.

Where to find the correct pressure:

• A sticker on the inside edge of the driver's door or door jamb (the most reliable source)
• The owner's manual (fuel economy and handling chapter)
• Sometimes printed inside the fuel-filler door flap

A typical passenger car runs around 220–250 kPa (32–36 psi) front and rear, though some vehicles use different pressures front to back, and some high-load or performance vehicles specify higher values. Always use your car's placard figure.

Check pressure when tires are cold. Driving heats the air inside the tire, expanding it and raising the reading by 20–40 kPa (3–6 psi). Cold means the car has been parked for at least three hours, or driven less than 1.5 km at low speed. If you must check after driving, add about 4 psi to the placard value as a rough correction — but always re-check cold later to confirm.

To check and adjust pressure, remove the valve stem cap, press a quality gauge firmly onto the stem, and read the value. Add air at a service station or with a portable compressor. If the pressure is too high, press the small pin inside the valve stem with the gauge edge or a key tip to release air in short bursts.

Check tire pressure at least once a month and before any long trip. Tires lose roughly 1 psi (7 kPa) for every 10 °C drop in ambient temperature as the seasons change.

Under-Inflation vs Over-Inflation — Effects and How to Spot Them

Both under- and over-inflation are harmful, just in different ways. The wear patterns they leave on the tread are a useful early-warning signal.

Under-inflation causes the tire to sag at the sides. The outer shoulders carry more load than the center, so both edges wear faster while the center tread stays relatively thick. You may also notice the car feels sluggish or drifts, fuel economy drops, and the steering feels vague. Severely under-inflated tires generate excess heat and can fail suddenly — a leading cause of blowouts at highway speed.

Over-inflation makes the tire balloon outward so the center of the tread bears the load. The center strip wears faster while the shoulders look nearly new. The ride becomes noticeably harsher, and the shorter contact patch reduces grip in emergency braking.

Uneven Wear Patterns and What They Mean

Even with correct pressure, tires can wear unevenly due to alignment, suspension, or driving issues. Reading the wear pattern gives you clues about what to fix before buying new tires — replacing without fixing the underlying problem just repeats the damage.

If you spot one-side or feathering wear, book a wheel alignment before purchasing new tires. Alignment is usually a quick workshop job costing 5,000–10,000 yen (roughly $35–70 USD) and can double the life of your next set of tires.

Age, Cracks, Bulges, and the Date Code — When to Replace Regardless of Tread

Tread depth is not the only reason to replace a tire. Rubber ages and hardens even when the tread looks deep, especially in hot climates or if the car sits unused for months at a time. Most manufacturers and safety bodies recommend replacing tires at or before 10 years from manufacture, and strongly advise inspection after 5 years even if tread remains.

How to read the date code (DOT code). On the sidewall you will see a string beginning with DOT followed by letters and numbers. The last four digits are the week and year of manufacture. For example, 2319 means the 23rd week of 2019. A tire with a code showing more than 6 years ago deserves close inspection; one over 10 years should be replaced regardless of how the tread looks.

Sidewall cracks (crazing). Fine surface cracking in the sidewall or at the base of the tread grooves is a sign of rubber oxidation and aging. Light surface craze is common and may be cosmetic, but deep cracks — especially those you can see into — indicate the tire structure is compromised. Replace it.

Bulges and blisters. A bulge on the sidewall means the internal cord structure has broken, usually from hitting a pothole or curb hard. The bubble is the outer rubber layer holding back air pressure — it can fail without warning. Any bulge means immediate replacement, full stop.

Punctures and repairs. A nail in the tread can often be safely plug-and-patched by a tire shop if it is in the central three-quarters of the tread and the tire has not been driven flat. Sidewall punctures cannot be safely repaired — replace the tire.

Monthly Tire Check — A Quick Routine That Takes Under Five Minutes

Building a short monthly habit catches problems early and keeps you safe. Do this every time you fill up fuel or at the start of each month — whichever is easier to remember.

• Walk around the car and look at each tire. Any obvious flat? Any visible object embedded in the tread?
• Check all four pressures with a gauge (and the spare if you have a full-size one). Adjust to placard spec cold.
• Crouch down and look at the tread wear indicators in the grooves. Are any indicators flush with the tread?
• Run your hand along the sidewall and look for cracks, cuts, or any bumps or bulges.
• Glance at the wear pattern across the tread width — is it even, or is one side clearly more worn?

If anything looks wrong, do not delay. Tires are not an area to manage with a wait-and-see approach. A tire failure at highway speed is one of the most dangerous situations a driver can face, and almost all of them are preventable with basic regular checks.

Summary — Key Numbers to Remember

You do not need to memorise everything. Keep these numbers on hand and you will handle 90% of tire decisions correctly.

• 1.6 mm — legal tread depth limit; replace at or before this point (4 mm for winter tires)
• 3 mm — plan replacement soon; wet-weather performance drops significantly
• 6–10 years — tire age window for inspection and likely replacement regardless of tread
• Monthly — how often to check pressure and do a visual inspection
• Door placard — where to find your correct tire pressure (not the sidewall number)
• Cold tires — always check pressure before driving, or after 3+ hours parked

Related reading

• Brake System Basics
• Tire Rotation by Video

This article was prepared by the Car Care Lab editorial team for educational purposes, drawing on widely published service information, manufacturer guidance, and maintenance videos. Intervals, prices, and procedures are representative guides only — always follow your vehicle's owner's manual, and if you are unsure or the job affects safety-critical systems (brakes, steering, high-voltage EV components), have it done by a certified workshop.