Cold inflation pressure

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

Cold inflation pressure is the inflation pressure of tires as measured before a car is driven and the tires warmed up. Recommended cold inflation pressure is displayed in the owner's manual and on the Tire Information Placard attached to the vehicle door edge, pillar, glovebox door or fuel filler flap.

Cold inflation pressure is a gauge pressure and not an absolute pressure.

This article focuses on cold inflation pressures for passenger vehicles and trucks. The general principles are, of course, applicable to bicycle tires, tractor tires, and any other kind of tire with an internal structure that gives it a defined size and shape (as opposed to something that might resemble a very flexible balloon).

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.^[1] The number one cause of tire failure was determined to be under-inflation. Drivers are encouraged to make sure their tires are adequately inflated at all times.

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 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.

Variation of tire pressure with temperature

Daily temperature fluctuations can result in appreciable changes in tire pressure. Cold inflation pressure should therefore be measured in the morning, as this is the coldest time of day. This will ensure a tire meets or exceeds the required inflation pressure at any time of day.

Seasonal temperature fluctuations can also result in appreciable changes in tire pressure, and a tire that is properly inflated in the summer is likely to become underinflated in the winter. ^[2] Because of this, it is important to check tire pressures whenever the local seasons change.

Variation of tire pressure with altitude

Atmospheric pressure will decrease around 0.5 psi for every 1000 feet above sea level. As a vehicle descends from a high altitude location, the absolute pressure inside the tire remains the same, but the atmospheric pressure increases; therefore the gauge pressure will decrease.

Take for example a vehicle which had its cold inflation tire pressure set near Denver (altitude 5300 feet), and is descending towards Los Angeles (altitude 300 feet). The tires could become underinflated by as much as 2.5 psi.

Cold inflation pressure should therefore be readjusted after any significant changes in altitude.