Cold inflation pressure is the inflation pressure of tires before a car is driven and the tires (tyres) warmed up. Recommended cold inflation pressure is displayed on the owner's manual and on the placard (or sticker) attached to the vehicle door edge, pillar, glovebox door or fuel filler flap. Most passenger cars are recommended to have a tire pressure of 2.1 to 2.4 bars (210 to 240 kPa; 30 to 35 psi) when not warmed by driving.[1] A 2001 NHTSA study found that 40% of passenger cars have at least one tire under-inflated by 0.4 bars (6 psi) or more.[2] Drivers are encouraged to make sure their tires are adequately inflated, as under inflated tires 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 tire. Excessive pressure, on the other hand, will lead to impact-breaks, decreased braking performance, and cause increased wear on the center part of the tread surface.
Tire 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 (kPa), which is an SI unit.
Ambient temperature affects the cold tire pressure. Cold tire absolute pressure (gauge pressure plus atmospheric 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. If the volume of the tire remains constant, a 1% increase in absolute temperature results in a 1% increase in absolute pressure.
As an example, a tire is inflated to 2.2 bar (220 kPa; 32 psi) at an ambient temperature of 300 K (27 °C; 80 °F). If the absolute temperature of the air in the tire increases by 1% to 303 K (30 °C; 86 °F), the absolute pressure also increases by 1%. The absolute pressure is the sum of the ambient atmospheric pressure (approximately 1.01 bars [101 kPa; 14.7 psi] at sea level) and the gauge pressure, giving a total of 3.21 bar (321 kPa; 46.6 psi). This 3 K (3.0 °C; 5.4 °F) increase results in a change of .0321 bar (3.21 kPa; 0.466 psi). However, a similar tire inflated at an ambient temperature of 250 K (−23 °C; −10 °F) only needs to be warmed up by 2.5 K (2.5 °C; 4.5 °F) to see the same 1% increase in pressure.
Due to the temperature-dependent pressure changes, seasonal temperature fluctuations can result in appreciable changes in tire pressure.[1]
(Assuming standard sea-level atmospheric pressure of 14.696 pounds per square inch or 101.33 kilopascals)[3]
