Cold inflation pressure


Cold inflation pressure is the inflation pressure of tyres before the car is driven and the tyres warmed up. Recommended cold inflation pressure is displayed on the owner's manual and on the placard attached to the vehicle door edge, pillar, glovebox door or fuel filler flap. 40% of passenger cars have at least one tyre under-inflated by 6 psi or more. Drivers are encouraged to make sure their tyres are adequately inflated, as under inflated tyres can greatly reduce fuel economy, increase emissions, cause increased wear on the edges of the tread surface, and can lead to overheating and premature failure of the tyre. Excessive pressure, on the other hand, may lead to impact-breaks, decreased braking performance, and cause increased wear on the center part of the tread surface.
Tyre pressure is commonly measured in psi in the imperial and US customary systems, bar, which is deprecated but accepted for use with SI or the kilopascal, which is an SI unit.
Ambient temperature affects the cold tyre pressure. Cold tyre absolute pressure varies directly with the absolute temperature, measured in kelvin.
From physics, the ideal gas law states that PV = nRT, where P is absolute pressure, T is absolute temperature, V is the volume, and nR is constant for a given number of molecules of gas. To understand this, assume the tyre was filled when it was 300 kelvins. If the temperature varies 10%, the pressure varies 10%. So if the tyre was filled at 80 °F to 32 psi, the change would be 4.7 psi for this 30 Celsius degree change, or 0.16 psi per Celsius degree or 0.1 psi per Fahrenheit degree or 1 psi for every 10 Fahrenheit degrees. Using SI units, that would be 1.1 kPa/K.
Hence, for a tyre filled to 32 psi, the approximation usually made is that within the range of normal atmospheric temperatures and pressures:
Tyre pressure increases 1 psi for each 10 Fahrenheit degree increase in temperature, or conversely decreases 1 psi for each 10 Fahrenheit degree decrease in temperature and
in SI units, tire pressure increases 1.1 kPa for each 1 Celsius degree increase in temperature, or conversely decreases 1.1 kPa for each 1 Celsius degree decrease in temperature. For tyres that need inflation greater than 32psi it might be easier to use a Rule of Thumb of 2% pressure change for a change of 10 degrees Fahrenheit.
From the table below, one can see that these are only approximations:

Variation of tyre pressure with temperature in Fahrenheit and Celsius

Pressure at
20°C 		10 psi20 psi30 psi40 psi50 psi60 psi70 psi80 psi90 psi100 psi
104°F11.7 psi22.4 psi33.1 psi43.7 psi54.4 psi65.1 psi75.8 psi86.5 psi97.1 psi107.8 psi40°C
86°F10.8 psi21.2 psi31.5 psi41.9 psi52.2 psi62.5 psi72.9 psi83.2 psi93.6 psi103.9 psi30°C
68°F10.0 psi20.0 psi30.0 psi40.0 psi50.0 psi60.0 psi70.0 psi80.0 psi90.0 psi100.0 psi20°C
50°F9.2 psi18.8 psi28.5 psi38.1 psi47.8 psi57.5 psi67.1 psi76.8 psi86.4 psi96.1 psi10°C
32°F8.3 psi17.6 psi26.9 psi36.3 psi45.6 psi54.9 psi64.2 psi73.5 psi82.9 psi92.2 psi0°C
14°F7.5 psi16.4 psi25.4 psi34.4 psi43.4 psi52.4 psi61.3 psi70.3 psi79.3 psi88.3 psi−10°C
−4°F6.6 psi15.3 psi23.9 psi32.5 psi41.2 psi49.8 psi58.4 psi67.1 psi75.7 psi84.3 psi−20°C
−22°F5.8 psi14.1 psi22.4 psi30.7 psi39.0 psi47.3 psi55.5 psi63.8 psi72.1 psi80.4 psi−30°C
−40°F4.9 psi12.9 psi20.8 psi28.8 psi36.8 psi44.7 psi52.7 psi60.6 psi68.6 psi76.5 psi−40°C
Pressure at
20°C 		69 kPa138 kPa207 kPa276 kPa345 kPa414 kPa483 kPa551 kPa620 kPa689 kPa
104°F81 kPa154 kPa228 kPa301 kPa375 kPa449 kPa522 kPa596 kPa670 kPa743 kPa40°C
86°F75 kPa146 kPa217 kPa289 kPa360 kPa431 kPa502 kPa574 kPa645 kPa716 kPa30°C
68°F69 kPa138 kPa207 kPa276 kPa345 kPa414 kPa483 kPa551 kPa620 kPa689 kPa20°C
50°F63 kPa130 kPa196 kPa263 kPa329 kPa396 kPa463 kPa529 kPa596 kPa662 kPa10°C
32°F57 kPa122 kPa186 kPa250 kPa314 kPa378 kPa443 kPa507 kPa571 kPa635 kPa0°C
14°F52 kPa113 kPa175 kPa237 kPa299 kPa361 kPa423 kPa485 kPa546 kPa608 kPa−10°C
−4°F46 kPa105 kPa165 kPa224 kPa284 kPa343 kPa403 kPa462 kPa522 kPa581 kPa−20°C
−22°F40 kPa97 kPa154 kPa211 kPa269 kPa326 kPa383 kPa440 kPa497 kPa554 kPa−30°C
−40°F34 kPa89 kPa144 kPa199 kPa253 kPa308 kPa363 kPa418 kPa473 kPa527 kPa−40°C