I have had the same problem bad gas mileage too.
I went and got at trueflow air filter and it helped alot.
go to www.trueflow.com and check it out. I think its better than K&N air filters. More air will get you better mileage.
hope this helps.

 

i just hit 21mpg in my 01 3.9 when i filled up the other day. That was on about 80% highway with cruise on 60, 20% city driving. I was on a little lower than 1/4 tank with 305 miles on the trip odometer.

 

i get roughly 18-20mpg from mine depending on just how much highway driving I do. I bet mine is about 80% city driving. Almost always get 325-330 a tank.

 

It depends on where you live, the air quality, how you drive, where you drive (highway/town)... it's hard to pinpoint why your mileage is low, but I use to get roughly the same mpg on my quad cab before Dual Exhaust and my redneck intake.