Properly inflated tires reduce resistance between the road and the tire, allowing the engine to run smoother. The resistance caused by under-inflated tires causes the engine to burn more fuel as it works harder to propel the vehicle forward.
Tires inflated to the recommended range of pounds per square inch may improve the vehicle's gas mileage by 3.3 percent, according to the U.S. Department of Energy. Even a drop of 1 pound per square inch in the four tires can reduce gas mileage by 0.3 percent. Most vehicles have a sticker that lists the recommended tire pressure either in the glove box or on driver's side door jamb.