2001 Chevy Cavalier low fuel milage

I am getting really low fuel mileage. I understand that it can have many causes but i am thinking mechanical. The reason why is because it was getting better mileage when i got it. also when starting off from a stop it is sluggish like it is getting to much fuel and flooding it self. It has not though any codes (or at least any that can be read by a OBD 2 scanner). It is still running and drive but doesn't like to move. When going down the freeway (70mph) i can see the fuel gauge move. Form were i live to portland and back it takes about half a tank. It should only take about 1/4 tank. Any help would be nice. My old car did the same thing and it was the mass air flow sensor that went out but it though a code that i can read. According to my local parts store there is no mass air flow sensor in this car. Again any help would help.