99 1.6L GXE gets 28 mpg hywy. Why so low???

When I first got the car from Atlanta Georgia @ 60K, I drove it back and it got like 35 miles per gallon going about 75 - 80 mph. I live in Illinois and have since driven it to Colorado and Arizona and have only been able to get about 27 - 29 MPG going about 75 - 80 HWY. It now has 90K on it. I was told by a shop that the Catalytic converter was clogged and so I replaced that along with the O2 sensors. However that has not increased the MPG. It has a Cherry Bomb Turbo low back pressure muffler now on it and a high airflow filter as the only mods. I have thought of everything and it just seems helpless. The only thing I can think of now is this plastic unercarriage that is hanging slightly not attached under the engine might affect the aerodynamics. New spark plugs and wires and rotor caps. Had a dyagnostic test done on the car and everything is running the way it should. Im just pissed off because gas prices are high and this car is a 1.6 liter slow ass engine, Im about ready to sell this car and get something with the same fuel economy and more horsepower, thinking like a 96 Infiniti G20. Any help is appreciated.