Tire Tread Depth and Pressure: The Monthly Check

Step 1: Check pressure cold

NHTSA calls proper tire pressure the most important part of tire maintenance, affecting safety, durability and fuel use. Its advice:

1. Find the right number on the Tire and Loading Information label, usually on the driver's door post (the "B-pillar"), sometimes on the door edge, or in the owner's manual. It is given in psi and kPa.
2. Check when cold: the car should not have been driven for at least three hours.
3. Check all tires, including the spare.
4. Adjust: let air out by pressing the valve core gently with the gauge edge if too high, or add the missing pounds if too low.
5. Recheck that all tires are at the specified pressure; some vehicles specify different front and rear pressures.

Keep a tire pressure gauge in the car. NHTSA notes a tire can lose pressure suddenly after a pothole or curb strike.

Why pressure matters

Underinflation is a leading cause of tire problems. NHTSA lists poor maintenance, including too little air and skipping rotations, as a cause of flats, blowouts and tread separation. It also says proper inflation can save fuel and that properly inflated tires can extend average tire life.

The tire pressure warning light

Cars, light trucks and vans from model year 2008 onward must have a tire pressure monitoring system (TPMS). NHTSA explains that the light comes on only when a tire is significantly underinflated, so it is not a substitute for the monthly check. A light that flashes for about a minute at startup and then stays on indicates a system malfunction; have the dealer inspect it.

Step 2: Check tread depth

NHTSA says tires are not safe and should be replaced when tread is worn to 2/32 of an inch. Three ways to check:

• Wear bars: raised bars across the bottom of the grooves. When the tread is level with them, replace the tire.
• Penny test: put a penny in a groove with Lincoln's head upside down and facing you. If you can see the top of his head, replace the tire.
• Tread depth gauge: an inexpensive tread depth gauge reads in 32nds of an inch, so you can track wear over time and spot uneven wear early.

Measure in several grooves across the tire and at a few points around it. Wear that differs from one edge to the other or from front to back is a sign to check alignment, pressure or rotation.

Step 3: Look for damage

While you are at each tire, look for cuts, cracks, bulges, embedded objects and uneven wear. NHTSA lists physical damage and irregular wear from underinflation or overloading as reasons to stop using a tire. Bulges in the sidewall are especially serious.

Rotation, balance and alignment

NHTSA recommends following the owner's manual for rotation intervals and patterns, and notes that where the manufacturer recommends rotation, it is typically every 5,000 to 8,000 miles or sooner if uneven wear appears. Vehicles with different front and rear sizes may not be able to rotate. New tires should always be balanced when installed, and alignment keeps the car tracking straight and helps tires last.

A five-minute monthly routine

1. Before driving, read the placard pressure.
2. Check and adjust all tires and the spare.
3. Do a penny test or gauge check on each tire.
4. Walk around looking for damage.
5. Note the date and readings, so slow leaks show up.

Pair it with the tire age check once a year.

Common questions

Where do I find the right tire pressure?

On the Tire and Loading Information label on the driver's door edge or door post, or in the owner's manual. NHTSA stresses this is the correct pressure, not the figure on the tire.

Can I check pressure after driving?

Readings are higher when tires are warm. NHTSA's advice: if you think a tire is low after driving, fill it to the cold placard pressure, then recheck when cold.

Does the tire pressure light mean my tires are fine when it's off?

No. NHTSA says the warning comes on only when a tire is significantly underinflated, so a monthly gauge check is still needed.

Why does the pressure light come on on cold mornings?

Cold air lowers tire pressure. NHTSA says a light that comes on in the cold and goes off as tires warm likely means pressure is marginally low; check and adjust.