Ive got an 07 with the 4.7, with 3" back to a y-pipe and 2.5" strait out the back. Still have cats, just no muffler. It sounds great, everyone says that it is the best sounding truck theyve ever heard. The only thing im not too impressed with is the overwhelming drone in the cab. Its fun in town, but on trips and interstate it is horrible. I dont want to put any dynomat in really, it sounds too time consuming.
I am getting an average of 16.5 in the city, and 18.4 going 80 on the interstate (K&N CAI helped alot) but i was wondering about getting a muffler put in. How much would it help with mileage and i know it'll help with the drone, i had the super 40 on my ford.

 

u need some back pressure. most dodges lose thier gas mileage with their straight pipes (no muffler).

Magnaflow or Aero AT3030 is a way to go because their free flowing muffler. they should increase by .5 mpg to 2 mpg.

 

Sounds to me like you are doing pretty good, 18.4 at 80 MPH. I wish my truck saw 18 MPG.