Have you ever peddled a bicycle with under-inflated tires and then noticed the difference in effort after you inflated the tires to their proper pressure? Your automobile feels the same way.

How much of a difference in gas mileage does tire pressure make?

Under-inflated tires lower gas mileage by 0.4% for every 1 psi (pounds per square inch) drop in the pressure of all four tires. The fueleconomy.gov web site estimates that drivers can save about 3.3% off their gas costs by keeping their tires properly inflated.