Check your tires air pressure when the tires are cool because heat (from driving or the ambient temperature) can increase tire pressure by several pounds per square inch (psi). Dont inflate the tires to the pressure stamped on the tires sidewall; thats the maximum inflation pressure. Tires lose pressure naturally typically 13 psi per month because a tires sidewall is permeable. So be sure to check your cars tire pressure at least once a monthespecially before a long trip. An increasing number of new vehicles today come with tire-inflation kits instead of spare tires; some vehicles are equipped with run-flat tires and no spare tire.