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%, 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%.
Keep Tires Properly Inflated
You can improve your gas mileage by up to 3.3% by keeping your tires inflated to the proper pressure. Under-inflated tires can lower gas mileage by 0.3% for every 1 psi drop in pressure of all four tires. Properly inflated tires are safer and last longer.1
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.
Use the Recommended Grade of Motor Oil
You can improve your gas mileage by 1%–2% 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%. Using 5W-30 in an engine designed for 5W-20 can lower your gas mileage by 1%–1.5%. Also, look for motor oil that says “Energy Conserving” on the API performance symbol to be sure it contains friction-reducing additives.1
Replacing a Clogged Air Filter on Modern Cars Improves Performance but Not MPG
Replacing a clogged air filter on vehicles with fuel-injected, computer-controlled gasoline engines—such as those manufactured from the early 1980s to the present—or diesel engines does not improve fuel economy, but it can improve acceleration.
Replacing a clogged air filter on an older vehicle with a carbureted engine can improve both fuel economy and acceleration by a few percent under normal replacement conditions.
Source : @ http://www.fueleconomy.gov/feg/maintain.jsp