It's basic common sense that any car will get better gas mileage when it is running in perfect condition. That means that making sure to keep the car tuned up and to address and to have address things like a check engine warning light as soon as possible if it comes on. A faulty oxygen sensor, for example, is one of the worst offenders and can reduce gas mileage by a whopping 40% in some cases. While a tune-up or repair work may involve a trip to the dealer or a qualified service station, there are ways for the car owner to make sure that his or her vehicle is running at its optimum level that don't cost anything.
Studies show that, on average, car tires on American roads are 5-15 psi below the pressure recommended by the manufacturer. The U.S. government website Fueleconomy.gov says that gas mileage is reduced by three-tenths of one percent for each psi below a tire's recommended. Operating with a 10 psi deficit in tire pressure, therefore, translates to a 3% loss in gas mileage. If you tried to buy a fuel efficient car that gets 30 miles to the gallon, you'd be giving back 0.9 mpg by not keeping your tire pressure topped off. To think of it another way, that means you have to buy 3% more gas than you would otherwise. It's free to stop at most gas stations to check your tire pressure and aid air if needed, but it can be expensive if you don't. Tires will also last longer when kept at the recommended pressure, reducing your costs there as well.
Some are going so far as to recommend filling tires with nitrogen instead of ordinary compressed air. By itself, this doesn't have any effect on mileage whatsoever. It does, however, slow the natural pressure loss that comes with air leaking out of tires normally. It's a little harder for nitrogen to escape the tire, so you'll have to check the pressure less frequently. Nitrogen tire fills are generally not free though and aren't available everywhere.