7 MPG city 8 highway!!

I get 7.0 mpg city, 8.0-8.3 highway!! No matter how i drive( heavy foot on gas pedal or light foot) i am getting almost the same MPG!!!! Ive changed spark plugs! What other things should i check!?? EGR and Oxygen sensor could be the issue?? How can i know if the oxygen sensor needs changing if EGR needs changing?? Wouldnt o2 sensor work bad because EGR is bad?? Please help thanx