But then again I drive like a grandma and mostly on HW. Anyway, to properly calculate your fuel economy reset your trip meter after a fill up. Then at the next fill up take the milage on the trip meter and divided by how many gallon of gas you just bought to fill up the tank. The whole how many miles you get on a tank of gas is really not all the accurate.
Otherwise check your timing and replace spark plugs if not done recently. You might also want to check the A/F to see if your car is running rich.