Nissan - Maxima :: 2011 - Bad Gas Mileage

I have a 2011 Nissan Maxima SV that has poor gas mileage. I took it to Nissan to get checked out but they said there's nothing they could do because the car is showing no codes or no check engine light. I got my spark plugs changed, it seemed like it worked but a week or so later, it went back to how it was (the gas mileage). 2 weeks later, my mechanic checked the spark plugs in front of me, no rust or leaks on them. I burn a half tank of gas every 2 days and that's just driving local to work and back.