rolling your windows up with A/C on is the same as windows down with it off. Wind resistance is the difference that ofsets the A/C.

 

i get about 15 city 20 highway with only cat and muffler delete on my auto, just picked up triple dog tuner so it shuld go up

 

I have owned autos and manuals both over the years and the manuals always got better fuel economy. With my current truck , which is my first diesel, I have a 5 speed manual and I get 17-18 local driving and 20-22 highway. All the gassers I owned I never could get anything above 15 highway with an automatic and usually around 10-12 local driving. I personally have no desire to ever own a truck with an automatic again.