* Keep your engine tuned to its optimal condition. Fixing a car that has failed an emissions test or has a larger malfunction, such as a faulty oxygen sensor, can improve its gas mileage by an average of 4 percent, according to www.FuelEconomy.gov.
* Use an additive. Engine additives such as a product called Lubrilon, made by Virginia-based XcelPlus (www.xcelplus.com), coat internal engine parts with a thin yet resilient layer of engine protection that reduces wear and tear and improves fuel mileage. According to a study by the University of Utah and the U.S. Department of Energy, this product can increase a car's miles per gallon by 6 percent to 17.5 percent.
* Check your car's air filter. The air filter helps to prevent impurities from damaging a car's engine. Checking and possibly replacing the air filter may improve your car's gas mileage by as much as 10 percent.
* Use the recommended grade of motor oil. While engine additives like Lubrilon can increase your engine's performance and thereby improve gas mileage, using the correct grade of motor oil for your car can help as well. If you change the oil yourself, be sure to verify the correct grade in the owner's manual. If you get your oil changed at a shop, request the correct grade just to make sure that it's being used.
* Properly inflate your tires. Keeping your tires inflated to their proper pressure is not only a safe thing to do, it can help improve your gas mileage by as much as 3.3 percent. Like the correct grade of motor oil, you can find your tires' proper inflation rate in the owner's manual.