If you're milage is that bad, something could be very wrong. Does it run like shit/have any drivability issues? You're not driving around on flat tires are you?

And if you're considering taking it to the dealership for diagnosis, the last thing you want to do is throw a bunch of aftermarked parts on it. You'll ruin any chance you had of getting any "goodwill" service. They'll just tell you, "no wonder you get shit milage, look at all this aftermarket crap."

I would be seriously concerned if I were you.