forgive me.. i didnt see he had a 7th gen

 

Did you do basic stuff like: air filter, plugs and pcv valve. Check these things and inspect them? If you by any chance see if check engine light on? Scan it for codes .

 

I have the same car (03V6 auto), and I would probably have to really abuse the gas pedal to get 15mpg. I average 23mpg just driving around town (not heavy traffic, but not highway). NGK or Denso Long-Life Irridium plugs are recommended. My car had the Denso plugs in from the factory. If you have over 100k miles I would change them. The rear bank can be a little dificult, without the right tools.