I will probably get slammed for this, but here it goes....
I had a 2007 Silverado Hd 2500 with the 6.0 and 4 speed. Traded for a 2010 GMC Hd2500 with the six speed....everyone said I would get better mpg with the newer truck. I did....went from 6.9 all the way to 7.2 when towing....wow!
I really thought I liked the GM products when I owned them, but last March I pulled my son's camper with his hemi....what a difference. I was so impressed, I bought a new 2013 Ram Hemi in May.....happy I did it.
Mileage towing is 9.6 to 10.2, ride is smoother by a bunch, truck seems more solid, and the 6 speed is a lot more responsive and does not down shift wildly as the GMC did.
Not saying thatbGM doesmnot make a good truck and I was a GM owner for a long time....just noting that you might want to look around at what the other manufacturers have put together.
I drove a couple of F250s and did not like the ride and the 6.2 did not seem to have enough power....maybe gearing??
BTW, I never got 14 mpg with any GM tduck hiway.....13.8 was the closesf I ever got. I continually get 19+ hiway with the 2500 ram hemi....
Maybe the new 2014s are better from GM....I dunno