Inflate to the Max PSI on the Sidewall? Here's Why Not

The number in plain sight

You're at the air pump. You crouch down, look at the tire, and there it is in raised letters: MAX PRESS and a number. It's right there on the tire, so it must be the number to fill to.

It isn't. That number is a ceiling, set by the tire maker for that tire on any vehicle. The number your car needs is somewhere else, and it's almost always lower. Here's where to find it, why the two differ, and what happens if you fill to the maximum.

Two numbers, two different jobs

USTMA puts it plainly: the pressure on the tire's sidewall is the maximum allowed in the tire, irrespective of the vehicle. Follow the vehicle maker's recommendation on the tire placard, certification label or owner's manual instead. The federal government's fueleconomy.gov says the same thing in fewer words: do not use the maximum pressure printed on the tire's sidewall.

Where your number actually lives

Open the driver's door and look at the edge of the door or the post it closes against. You're looking for the Tire and Loading Information placard.

That label isn't optional. Federal Motor Vehicle Safety Standard No. 110 requires a placard permanently attached to the driver's side B-pillar (or the edge of the driver's door if the vehicle has no B-pillar), showing:

• the original tire size
• the vehicle maker's recommended cold tire inflation pressure for the front, rear and spare tires

So the placard can list different pressures front and rear, and a separate pressure for the spare. Set each position to its own number. NHTSA's TireWise guidance also points drivers to the placard and the owner's manual for the right pressure.

Why the vehicle maker's number is lower

The tire maker doesn't know which car its tire will end up on. The same size might go on a light sedan or a heavier crossover. So the sidewall gives the limit the tire itself can take.

The vehicle maker knows the car. USTMA notes that the placard pressure is based on a number of factors, such as load, and may not be the same as the maximum on the sidewall. That's the number that matches your car's weight, handling and ride the way it was designed.

What filling to the max does

On most cars, filling to the sidewall maximum means running well above the recommended pressure. The effects:

• Uneven wear. Michelin notes that too much pressure can cause the center of the tread to wear faster than the edges.
• A different feel. An over-inflated tire is stiffer, so the car rides and handles differently from how its maker set it up.
• Even higher when hot. Pressure climbs as tires warm up on the road and in the sun, so a tire filled to its maximum cold starts above it once warm.

Try that last one here. Enter a cold pressure and a temperature change:

Don't swing too far the other way

Under-inflation isn't the fix. Low pressure has its own costs: fueleconomy.gov notes that under-inflated tires lower gas mileage by about 0.2% for every 1 psi drop in the average pressure of all four tires. The goal is the placard number, not "lower is gentler."

Your TPMS light helps, but only as a backstop. It's designed to warn at a large loss, not to tell you the tires are at the placard pressure. Our guide to the TPMS light explains what it does and doesn't tell you.

How to set pressure the right way

1. Find the placard on the driver's door edge or door post, or check the owner's manual.
2. Check cold. USTMA says to check when tires are cold, meaning parked for at least three hours. First thing in the morning is easiest.
3. Use a gauge. Check all four, and the spare if it carries one.
4. Fill to the placard number for each position, front and rear.
5. Never exceed the sidewall maximum. On the tires your car came with, the placard number is at or below it.
6. Don't let air out of warm tires. NHTSA's summer guidance warns against it: pressure rises as tires heat up, and a warm tire bled down to the placard will be low once it cools.
7. Repeat monthly and before long trips. USTMA recommends checking at least once a month.

Our guide to tire pressure and temperature covers how much readings swing with the weather.

When the sidewall number does matter

The maximum isn't useless. It matters as a limit:

• Never fill above it, whatever the reason.
• If you've changed tires from the size or load type the vehicle came with, the right pressure may not be the placard number. That's a question for a tire professional, using the tire maker's load and inflation information. Our tire size decoder shows what each part of your size means.

The one-line version

The sidewall tells you the most the tire can take. The door tells you what your car needs. Fill to the door.

If a tire keeps drifting below the placard number, that's a leak worth finding, not a pressure setting to change. In South Florida, Extreme Tires checks pressure, finds leaks and services TPMS in Sunrise or by mobile van in the local install area. Need new tires instead? Shop by size or vehicle.