You should check the tire pressure frequently, I check mine on a month to month basis, when they are a few pounds below, or a few pounds over, my gas mileage is affected. With gas prices being so high these days, also when you drive don't stomp on the gas pedal, that makes you use more gas than you normally would. Another thing, you should make sure you vehicle is properly tuned, ie plugs, wires, air filter. Another thing you could try, and I do this myself every other month, is an intake flush. If you have any friends that are mechanics, have them do it, its much cheaper than taking it to the shop where they can charge you anywhere from 50 to 100 dollars for the job. All it takes is a can of carb and choke cleaner, sprayed into the intake, let it sit, so it cleans up the heads and the intake, and then start it up, it will have difficulty starting, but run it, rev it, and it should blow out alot of the carbon build up in your intake, that usually helps keep the intake clean, and more efficient on gas