You can improve your gas mileage by up to 3.3 percent by simply keeping your tires inflated to the proper pressure, according to Fueleconomy.gov. Underinflated tires can lower gas mileage by 0.3 percent for every pound-per-square-inch drop in pressure of all four tires. (You can get a tire-pressure gauge at any hardware store or auto store for about $10.) Check your vehicle's owner's manual or the door jamb for the proper level of inflation (not the tire itself, which shows the maximum tire inflation pressure), and be sure to check the tire pressure when the tires are cold, as internal pressure increases when the car has been on the road for a while and the tires heat up.
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