Charm's math is correct and your gas mileage is terrible. You should go twice as far on a tank of fuel. It will pay you to correct your problem(s).

Things to check-
1) Is the neighbor kid siphoning gas out of your tank at night?
2) Air filter clean?
3) Fuel injectors clean? (I use Techron)
4) Timing belt correctly installed (cam timing correct).
5) Ignition timing correct and timing advance working.
6) Fuel leaks? May leak only when running.
7) Obstructed exhaust? You should be able to feel the individual bursts of exhaust at the tailpipe opening with your hand when the car is idling.
8) Airbox thermostat closing when warm (not pulling hot air into the airbox).
9) Oxygen sensor in specifications?

If I were you, I'd check on ALL these things, even after you find one or two problems and correct them.

Other things that will cut mileage a little
1) Tires wider than necessary, or under-inflated.
2) Heavy right foot.
3) Excess cargo being hauled around.

Good Luck!
Manolo