According to the laws of physics, a closed system’s gas volume and pressure only remains constant as long as its temperature remain constant. Since a tire’s air chamber is a closed system filled with a gas, tire inflation pressures react (rise or fall) in line with changes in their operating temperature. As a general rule, tire pressures change 2% (about 1 pound per square inch (psi) for car tires) for every 10° Fahrenheit change in a tire’s air chamber temperature.
Vehicle manufacturer recommended tire pressures are to be checked and set when “cold.” These conditions are typically defined as, in the morning before ambient air temperatures rise, the vehicle is driven or exposed to direct sunlight; all of which will cause a temporary artificial buildup.
What contributes to changes in operating conditions?
Ambient air temperatures fluctuate daily and seasonally. In most locations, afternoon high temperatures are about 20° Fahrenheit warmer than morning lows, and the range of average summertime temperatures is about 40° Fahrenheit warmer than those experienced in wintertime. These changes result in tire pressures predictably dropping overnight and during the fall and winter seasons.
A tire’s bending, stretching and deflecting when rolling converts tire motion into heat, also temporarily increasing inflation pressures. It is common for hot tire inflation pressures to increase 4 to 6 psi above “cold” values when driven on hot roads and/or in high ambient temperatures.
Tires absorb heat when exposed to direct sunlight, causing their air chamber temperature to increase. A tire’s block color makes it common for parked tires exposed to direct sunlight to have inflation pressures 4 to 6 psi higher than tires sitting in the shade.
NOTE: While the influence of these temporary increases is additive, even when combined, they do not increase tire inflation pressures sufficiently to cause a properly inflated tire to fail catastrophically in a blowout.
How to determine the appropriate tire pressure for the current conditions:
If current conditions meet the “cold” definition, always set tire inflation pressures at the vehicle manufacturer’s recommended “cold” tire pressures provided in its owner’s manual or listed on the Tire and Loading Information placard on the driver’s doorjamb.
In tire pressures are checked and reset in other than “cold” conditions, inflation pressure compensation requires comparing the “current” conditions to morning’s “cold” conditions.
Considering the following adjustments before resetting tire pressures in conditions other than “cold” to accommodate temporary temperature differences:
- Set pressures 2 psi above recommended during afternoon/early evening hours
- If tires are warm from being driven, set pressures 4 psi above recommended in the morning and 6 psi if checked and set in the afternoon.
- If tires are warm/hot from being driven and excessively high inflation pressures are found, do not bleed “hot” inflation pressures to less than 6 psi above their “cold” recommendation.
- For vehicles parked in heated shops or attached garages during wintertime, set tire pressures 1 psi above the placard recommendation for every 10° Fahrenheit temperature difference between the warm shop/garage and the cold outdoor temperatures.
While these minor adjustments help compensate for temporary temperature differences, it is suggested that inflation pressures be rechecked, and reset if necessary, the following morning when conditions again meet the “cold” definition.
NOTE: Tire pressures set at the vehicle manufacture’s recommended “cold” psi during temporarily high/hot ambient conditions will be underinflated when measured in “cold” conditions the following morning. In some cases this may cause the vehicle’s Tire Pressure Monitoring System (TPMS) to illuminate the dashboard warning light. Correctly compensating for current conditions will minimize the possibility of the TPMS from issuing a low-pressure warning.