some of y'all aint gonna like this,
But you'll just have to deal with it. Just a second, let me check my flame retardant suit for integrity. On a recent trip of 1,100+ miles round trip, I logged an amazing 22. 7 mpg, hand calculated and checked against my Magellan. My CTD is very mildly modified, including: 19. 5" Alcoas, with 225/70 radials, 4" exhaust, aFe magnum FORCE intake, and a Superchips programer, set to the 'no towing' setting for the moment. It takes about 5 minutes to reprogram, and the unit stays in the glove box, so it's not a problem to go from 1 setting to another. I use Amsoil 5W-30 and a full flow/bypass system. I also have a combo 5th wheel/gooseneck hitch mounted in the bed, behind an aluminum diamond plate toolbox, and the factory installed class V hitch at the rear. I run with no tailgate, and I almost always run speed limit "+5", nearly everywhere I go, where the limit is 55 or higher.
I have a slushbox 47RE, totally stock, and 3. 55:1 ring & pinion, also factory stock. To account for the differences in mileage and performance between 2 seemingly identical trucks, both being driven in approximately the same way and in similar terrain and weather conditions, would be an R&D project few of us could afford. These are machines, built and assembled by humans. It logically follows that no 2 are exactly alike, except possibly in outward appearance.
I modified my truck wih an eye toward improved performance, and to me, improved mileage was a part of the equation. So far, everything I've done has helped. I don't drive like a little old man going to church, but I don't do smoke shows or make smoky burnouts, either. Tires and fuel are too expensive for that, and I have to replace 6 tires, not just 4.
Over the past 35 years, I've learned that making horsepower, torque, and mileage on a street driven vehicle, whether gasser or diesel,is a series of small steps, not just 1 or 2 giant leaps. There's no such thing as a 'mechanic in a can', and there are a lot of expensive goodies out there, many advertised and sold through this website, that offer dubious improvements in return for funds expended.
Back to the point, I get pretty consistent 17 mpg around town. Of course it drops when I hook my TT to it, but I haven't towed with the Superchips installed, yet. You guys can believe my numbers, or not. I double checked them because at first I was not believing them myself. I have no reason to brag about it, because I'm the one paying for the fuel.