Planning to get brand new 2016 Shaker Cascadia EVOLUTION

Hi Everybody!
Currently my trucks have Cat C-15 engines. Good engine I can't complain, powerful, reliable. But fuel mileage is about 5.7 - 6 when I drive it and worse when I have my drivers in my trucks (trucks turned down at 68 MPH). I want good fuel mileage especially now when freight is cheap. I mean ridiculously CHEAP! I was always fan of Cat engine and never believed that new trucks (engines) are getting much better MPG.... Until recently my 2 good buddies got Evolutions... I couldn't believe what that truck gets. Hauling the same load (weight of load is around 44K lbs) following each other at the same speed 68 MPH from Missouri thru Memphis to Panhandle Florida. He got almost 8 MPG and I barely squeezed 5.2 out of my beautiful Pete 379. Another friend gets constantly 8-9 MPG. After that I made my decision I am buying Crapcadia Evo, this is moneymaker. Now I have a dilemma. What kind of engine should i get DD13 or DD15, transmission auto DT12 or 13 manual, which gear ratio will give me the best MPG? FYI I run all states but 80% of the time my trucks east of I-35. Most of the loads are heavy. Thanks in advance.

 

He has DD15 with auto tranny, I think 2.93 rears. Last week he picked up a load (about 20-25k lbs) in Atlanta and headed back home to FL. Flat route. Was driving slow like a grandma 63 all the way. He said he has got 10+ MPG. I thinks it's amazing! Money he saves on fuel covers payments for brand new truck with warranty. Honestly I don't want to get auto tranny but hey if gets better fuel mileage then why not?.. I just want to know for sure that auto is more efficient compare to manual. Are there any comparison tests or somebody who drove both auto and manual?