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How much does tire pressure drop when it gets cold?

Predict your tire pressure after a change in outside temperature, and find how much to inflate in a warm garage so the tires are right when you drive out into the cold.

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Results

Pressure at the new temperature

30.3

More than 10% below: top up the tires
Change in pressure
-4.7
Pressure to set now so it reads the measured value at the new temperature
40.2
Change per 10 °F
0.94

The first cold morning of fall

Every autumn, many drivers find the tire pressure warning light on during the first cold morning. The tires have not sprung a leak: the air inside them has simply cooled and contracted. The amount is easy to predict with the gas law.

The formula

At constant volume, the absolute pressure of a gas is proportional to its absolute temperature:

P₂ = (P₁ + P_atm) × T₂ ÷ T₁ − P_atm

  • P₁ is the gauge reading at the first temperature.
  • P_atm is atmospheric pressure, about 14.7 psi (1.013 bar) at sea level.
  • T₁ and T₂ are absolute temperatures: °F + 459.67 gives degrees Rankine, °C + 273.15 gives kelvins.

A tire’s volume does change a little with pressure, but the effect is small enough to ignore.

Example

You set your tires to 35 psi in a 70 °F garage, and the next morning it is 20 °F outside:

  • Absolute pressure: 35 + 14.7 = 49.7 psi
  • Temperatures: 529.7 °R and 479.7 °R
  • New absolute pressure: 49.7 × 479.7 ÷ 529.7 = 45.0 psi
  • New gauge reading: about 30.3 psi, a drop of 4.7 psi

That matches the rule of thumb of about 1 psi for every 10 °F.

Inflating in a warm garage

If you check your tires indoors at 70 °F but will drive at 20 °F, set them higher so they reach the recommended value outside. In the example, inflating to about 40 psi in the garage gives 35 psi in the cold. The calculator shows this value on the third line.

Good habits

  • Check the pressure once a month and before long trips, with the tires cold: before driving or after at least three hours parked.
  • Use the pressure on the sticker inside the driver’s door, not the maximum printed on the tire sidewall.
  • Do not let air out of hot tires after driving. The pressure rises a few psi as they warm up, and that is expected.
  • Under-inflated tires wear faster at the edges, increase fuel consumption and braking distance, and can overheat.

Since 2007, new cars sold in the United States must have a tire pressure monitoring system that warns when a tire is 25% or more below the recommended pressure. A light that goes off after a few miles of driving usually means the tires are just under the limit and need topping up.

Frequently asked questions

How much does tire pressure drop in the cold?

About 1 psi for every 10 °F of temperature drop. A tire set to 35 psi in a 70 °F garage reads about 30 psi on a 20 °F morning. The air has not leaked out; it has simply contracted.

Why does the calculator use absolute pressure?

The gas law works with absolute pressure, which is the gauge reading plus atmospheric pressure (14.7 psi at sea level), and absolute temperature in kelvins. Using the gauge reading directly would exaggerate the change.

When should I check tire pressure?

When the tires are cold, before driving or after the car has been parked for 3 hours. The recommended pressure is on the sticker inside the driver's door, not the maximum printed on the tire. Driving warms the tires and raises the pressure by a few psi.

When does the TPMS warning light come on?

US rules require the tire pressure monitoring system on new cars since 2007 to warn when a tire is 25% or more below the recommended cold pressure. That is why the light often appears on the first cold morning of autumn.

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Results are estimates provided for information purposes. They do not replace professional advice.

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