Physics:Cold inflation pressure

Short description: Inflation pressure of tires before a car is driven

A tyre pressure gauge displaying bar (outside) and pounds per square inch (inside)

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. 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. 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.

Variation of tire pressure with temperature in Fahrenheit and Celsius

(Assuming standard sea-level atmospheric pressure of 14.696 pounds per square inch or 101.33 kilopascals)

Pressure at 20 °C (68 °F)	10 psi	20 psi	30 psi	40 psi	50 psi	60 psi	70 psi	80 psi	90 psi	100 psi
104 °F	11.7 psi	22.4 psi	33.1 psi	43.7 psi	54.4 psi	65.1 psi	75.8 psi	86.5 psi	97.1 psi	107.8 psi	40 °C
86 °F	10.8 psi	21.2 psi	31.5 psi	41.9 psi	52.2 psi	62.5 psi	72.9 psi	83.2 psi	93.6 psi	103.9 psi	30 °C
68 °F	10.0 psi	20.0 psi	30.0 psi	40.0 psi	50.0 psi	60.0 psi	70.0 psi	80.0 psi	90.0 psi	100.0 psi	20 °C
50 °F	9.2 psi	18.8 psi	28.5 psi	38.1 psi	47.8 psi	57.5 psi	67.1 psi	76.8 psi	86.4 psi	96.1 psi	10 °C
32 °F	8.3 psi	17.6 psi	26.9 psi	36.3 psi	45.6 psi	54.9 psi	64.2 psi	73.5 psi	82.9 psi	92.2 psi	0 °C
14 °F	7.5 psi	16.4 psi	25.4 psi	34.4 psi	43.4 psi	52.4 psi	61.3 psi	70.3 psi	79.3 psi	88.3 psi	−10 °C
−4 °F	6.6 psi	15.3 psi	23.9 psi	32.5 psi	41.2 psi	49.8 psi	58.4 psi	67.1 psi	75.7 psi	84.3 psi	−20°C
−22°F	5.8 psi	14.1 psi	22.4 psi	30.7 psi	39.0 psi	47.3 psi	55.5 psi	63.8 psi	72.1 psi	80.4 psi	−30 °C
−40 °F	4.9 psi	12.9 psi	20.8 psi	28.8 psi	36.8 psi	44.7 psi	52.7 psi	60.6 psi	68.6 psi	76.5 psi	−40 °C
Pressure at 20 °C (68 °F)	69 kPa	138 kPa	207 kPa	276 kPa	345 kPa	414 kPa	483 kPa	551 kPa	620 kPa	689 kPa
104°F	81 kPa	154 kPa	228 kPa	301 kPa	375 kPa	449 kPa	522 kPa	596 kPa	670 kPa	743 kPa	40 °C
86 °F	75 kPa	146 kPa	217 kPa	289 kPa	360 kPa	431 kPa	502 kPa	574 kPa	645 kPa	716 kPa	30 °C
68 °F	69 kPa	138 kPa	207 kPa	276 kPa	345 kPa	414 kPa	483 kPa	551 kPa	620 kPa	689 kPa	20°C
50 °F	63 kPa	130 kPa	196 kPa	263 kPa	329 kPa	396 kPa	463 kPa	529 kPa	596 kPa	662 kPa	10 °C
32 °F	57 kPa	122 kPa	186 kPa	250 kPa	314 kPa	378 kPa	443 kPa	507 kPa	571 kPa	635 kPa	0 °C
14 °F	52 kPa	113 kPa	175 kPa	237 kPa	299 kPa	361 kPa	423 kPa	485 kPa	546 kPa	608 kPa	−10 °C
−4 °F	46 kPa	105 kPa	165 kPa	224 kPa	284 kPa	343 kPa	403 kPa	462 kPa	522 kPa	581 kPa	−20 °C
−22 °F	40 kPa	97 kPa	154 kPa	211 kPa	269 kPa	326 kPa	383 kPa	440 kPa	497 kPa	554 kPa	−30 °C
−40 °F	34 kPa	89 kPa	144 kPa	199 kPa	253 kPa	308 kPa	363 kPa	418 kPa	473 kPa	527 kPa	−40 °C

See also

Tyre pressure measuring instrument

• Direct TPMS
• Tyre-pressure gauge
• Tire-pressure monitoring system