Sounds like you are guestimating the fuel you are using by looking just at the fuel guage?
Best thing to do is fill the car up, do a weeks driving (or however long) and then refill it. Look at how many miles you've covered and how many gallons you've just put in and that'll tell you the real world fuel economy.
As an average you shoudl get 45mpg with mixed town driving - if on a good run i'll get 55-60mpg but spirited driving can take it to 40 (and less

)
When mine was doing some really poor economy it was all linked to an interim fault I was having which turned out to be the pressure switch causing the car to run rich and drink fuel!