Tire Pressure and Temperature: How Much PSI Changes

Same air, different reading

You set your tires to the door placard on Sunday afternoon. Monday morning, after the first cool front of the season, the gauge reads two psi low and the TPMS light is on. Nothing leaked. The air just got colder.

Temperature changes tire pressure every day, in both directions. Here's how much, why, and how to set your pressure so the number means something.

The rule of thumb: about 1 to 2 psi per 10°F

Three tire-industry sources put a number on it:

The direction is the same in all three. Warmer means higher, cooler means lower. NHTSA's winter guidance says it in one line: as the outside temperature drops, so does tire inflation pressure.

Treat the figure as a rule of thumb, not a measurement. The exact change depends on the tire, how much air is in it and how quickly the temperature moved.

Why it happens: the physics

A tire is a sealed container. Heat the air inside and its molecules move faster and push harder on the walls. Cool it and they push less. The amount of air hasn't changed, only its temperature.

The math is Gay-Lussac's law, which says the absolute pressure of a sealed gas rises and falls with its absolute temperature. For tires it needs two conversions, because a tire gauge reads pressure above the atmosphere and Fahrenheit doesn't start at absolute zero:

new psi ≈ (old psi + 14.7) × (new °F + 459.67) ÷ (old °F + 459.67) − 14.7

This assumes the tire's volume stays the same, which is close enough for an estimate. Here's what it gives for a tire set to 35 psi, used only as an example. Your vehicle's number is on the door placard.

That works out to a little under 1 psi per 10°F at this pressure, which sits at the low end of the industry range. Real tires can move more, which is why the sources give a range.

Driving heat is a separate effect

The table covers the air around the car. Driving adds heat of its own: the tire flexes as it rolls, the rubber warms up, and so does the air inside. A tire that's been on I-95 for half an hour reads higher than the same tire parked overnight, even on the same day.

That's why NHTSA defines cold as parked for at least three hours, and why its summer guidance says don't let air out of warm tires. Pressure rises as tires heat up; bleed them down to the placard while they're warm and they'll sit below it once they cool.

Florida situations where this shows up

The A/C garage to sunny driveway. A car filled in a cool garage and parked in the sun will read higher an hour later. It hasn't gained air. Set pressure before it goes out.

The gas station after a drive. The air machine is convenient, but your tires are warm. If a warm tire reads right at the placard number, it's likely a little low when cold. Add what's needed, then recheck in the morning.

The first cool front. Tires set on a warm day lose a couple of psi overnight. A tire that was already a bit low can cross the TPMS warning line. Our TPMS light guide explains what the light's behavior means.

Summer heat. Hot weather raises the reading, which can hide a tire that's low. Our guide to Florida heat and tires covers why pressure matters more, not less, in the heat.

How to set pressure so the number means something

1. Check cold. First thing in the morning, before the car moves, is easiest.
2. Use the door placard, not the sidewall. The number on the tire is a maximum, not the target.
3. Use a gauge. TPMS is designed to warn at a large loss, not a small one.
4. Set all four and the spare. NHTSA's guidance is to check monthly, spare included.
5. Recheck after a big temperature swing. A new season or a front is a good reminder.

Our guide to checking tire pressure walks through each step with a gauge.

When temperature isn't the explanation

Temperature moves all four tires by roughly the same amount at the same time. Tires also lose some air naturally: Bridgestone says about 1 psi a month on average, which is one more reason for the monthly check.

What temperature doesn't explain:

• One tire that reads lower than the others, again and again
• a tire that needs air every week
• a TPMS light that comes back soon after you've set all four

Those point to a leak: a nail, a valve stem or a bead that isn't sealing. Have it inspected. Extreme Tires repairs punctures after dismounting and inspecting the tire, and its TPMS service covers sensor scans and relearns, at the Sunrise shop or by mobile van in the local install area.