OP if it makes you feel any better, I'm getting about 16mpg's with mixed driving (1092mi on the clock) and while it will break it and hopefully get better, I am actively looking for solutions to improve this. That, and I drive like a nut. LOL
Check out NST pulleys, and anything else that will reduce your rotating mass/weight. I personally am planning on going with the URD TCAI, NST pulleys, URD Aftercat, and the MAF cal.....even though we have trucks, and they're not supposed to get good mileage, and they're not fast, blah, blah, blah....The same rules apply to any combustion engine. More air in/out, more efficiently is good.