Purchase an accurate tire gauge from your tire dealer, auto supply store or other retailer. If you must drive over one mile for air, before you leave home, measure the cold inflation pressure of each tire and record the actual underinflation amount for each tire. Upon arriving at the service station, measure each tire's inflation again and then inflate the warm tire to a level that is equal to this warm pressure, plus the cold underinflation amount. In this example, add 6 psi to the right rear tire to bring pressure up to match the other rear tire's warm reading. When the tire cools down, it should end up at the vehicle placard's recommended pressure.
Tires lose air normally through the process of permeation.
Changes in outdoor temperature can affect the rate at which your tire loses air. This change is more pronounced in hot weather. Generally speaking, a tire will lose one or two pounds of air per month in cool weather, and even more in warmer weather. Un derinflation is the leading cause of tire failure, so check inflation pressure regularly. It is normal for pressures to build up as a result of driving.