Tire pressure: the right PSI, and why the light comes on

Use the door label, not the sidewall

The sidewall number is a ceiling.

Your vehicle’s recommended pressure is on the Tire and Loading Information label on the edge of the driver’s door or, on newer vehicles, on the door jamb post called the B-pillar, and in the owner’s manual, in both psi and kilopascals. On a vehicle with no B-pillar, NHTSA says, the label may be on the rear edge of the driver’s door. The “maximum permissible inflation pressure” molded into the tire’s sidewall is the most air the tire should ever hold, and NHTSA stresses that it is not the vehicle manufacturer’s recommended pressure. Fueleconomy.gov gives the same warning: do not inflate to the sidewall maximum.

Check it cold, once a month

Cold means not driven for at least three hours.

NHTSA recommends checking all tires, including the spare, at least once a month, when the tires are cold — meaning the car has not been driven for at least three hours. Driving warms the tires and raises the reading. Keep a tire gauge in the car: hitting a pothole or a curb can drain a tire suddenly.

If you have to fill a warm tire

Fill to the cold number, then recheck.

If you have been driving and think a tire is low, NHTSA’s advice is to fill it to the recommended cold inflation pressure on the label. The warm tire may still end up slightly underinflated, but that is safer than driving on a significantly underinflated one. Recheck and adjust when you can take a cold reading.

What the warning light means

By federal rule, it lights at 25% below the recommended pressure.

All passenger cars, light trucks, and vans from model year 2008 on must have a tire pressure monitoring system (TPMS). Some use sensors inside the tires; others infer pressure from the wheel speed sensors. Federal Motor Vehicle Safety Standard 138 requires the low-pressure light to come on within 20 minutes once a tire is 25% or more below the recommended cold pressure, or below a minimum the standard sets for the tire type, whichever is higher. With a 35 psi recommendation, that is about 26 psi.

So the light is a last warning, not a gauge. NHTSA notes that it comes on only when a tire is significantly underinflated; a tire a few psi low is already costing fuel with the light still off. Keep checking monthly.

Why the light comes on when it turns cold

Cold air means lower pressure.

Tire pressure decreases in colder temperatures, which fueleconomy.gov lists among the reasons gas mileage falls in winter: lower pressure increases rolling resistance. That is why the warning often appears on the first cold mornings of fall. The fix is the same as always: check the tires cold and fill them to the pressure on the door label. If the light stays on, or one tire keeps losing air, have it inspected for a leak. Winter costs fuel in other ways too; see why gas mileage drops in winter.

What correct pressure is worth

Mileage, tire life, and grip.

Fueleconomy.gov estimates that keeping tires at the proper pressure improves gas mileage by 0.6% on average and up to 3%, and that mileage drops by about 0.2% for every 1 psi drop in the average pressure of all four tires. NHTSA says properly inflated tires can extend the average life of a tire by 4,700 miles. While you are at it, look at the tread: NHTSA says tires should be replaced when the tread is worn down to 2/32 of an inch.

For the other ways to save fuel, see how to improve gas mileage.