Keeping Your Car in Shape Keep Your Engine Properly Tuned Fixing a car that is noticeably out of tune or has failed an emissions test can improve its gas mileage by an average of 4 percent, though results vary based on the kind of repair and how well it is done. Fixing a serious maintenance problem, such as a faulty oxygen sensor, can improve your mileage by as much as 40 percent. Fuel Economy Benefit: 4% Equivalent Gasoline Savings: $0.13/gallon Keep Tires Properly Inflated You can improve your gas mileage by up to 3.3 percent by keeping your tires inflated to the proper pressure. Under-inflated tires can lower gas mileage by 0.3 percent for every 1 psi drop in pressure of all four tires. Properly inflated tires are safer and last longer. The proper tire pressure for your vehicle is usually found on a sticker in the driver's side door jamb or the glove box and in your owner's manual. Do not use the maximum pressure printed on the tire's sidewall. Fuel Economy Benefit: Up to 3% Equivalent Gasoline Savings: Up to $0.10/gallon Use the Recommended Grade of Motor Oil You can improve your gas mileage by 1–2 percent by using the manufacturer's recommended grade of motor oil. For example, using 10W-30 motor oil in an engine designed to use 5W-30 can lower your gas mileage by 1–2 percent. Using 5W-30 in an engine designed for 5W-20 can lower your gas mileage by 1–1.5 percent. Also, look for motor oil that says "Energy Conserving" on the API performance symbol to be sure it contains friction-reducing additives. Fuel Economy Benefit: 1–2% Equivalent Gasoline Savings: $0.03–$0.07/gallon NEW INFORMATION: Replacing a Clogged Air Filter on Modern Cars Improves Performance but Not MPG A new study shows that replacing a clogged air filter on cars with fuel-injected, computer-controlled gasoline engines does not improve fuel economy but it can improve acceleration time by around 6 to 11 percent. This kind of engine is prevalent on most gasoline cars manufactured from the early 1980s onward. Tests suggest that replacing a clogged air filter on an older car with a carbureted engine may improve fuel economy 2 to 6 percent under normal replacement conditions or up to 14 percent if the filter is so clogged that it significantly affects drivability. The effect of a clogged air filter on diesel vehicles will be tested in the near future. Note: Cost savings are based on an assumed fuel price of $3.36/gallon.