There's no hyperbole in saying that Cummins' B-series engine helped to redefine the diesel truck market in America. The Cummins light-duty truck engine itself was an evolution of previous designs, but the fact that Dodge opted to install it into consumer-grade pickups made it truly revolutionary. Dodge's Cummins diesel trucks aren't the most fuel-efficient vehicles in the world, but they do offer efficiency in a different league from any gas engine with the same performance.
Install a cold-air intake, high-flow air filter, aftermarket exhaust system, turbo down tube and underdrive pulleys. These upgrades are fairly universal, but they work particularly well on diesel engines where fuel efficiency is concerned. Diesel fuel economy is directly connected to torque output with a given amount of fuel, so anything you can do to enhance power without injecting more fuel is going to increase fuel efficiency.
Buy and install an aftermarket power programmer with a fuel efficiency setting. A power tuner will probably make the biggest difference in fuel efficiency without tearing into the engine and making major changes to the hardware -- and even then, you'll probably never match the power tuner's return on the dollar.
Lower the truck. Your truck's aerodynamic size and aerodynamic profile has at least as much impact on fuel economy as its weight, which you can't very well reduce without scrapping things that you probably want. Lowering the truck will reduce its aerodynamic cross-section (the size of the hole that it must punch through the air) and the amount of air that goes underneath the chassis.
Install a front air dam that very nearly scrapes the ground, and cover the bottom of the truck with a flat, sheet-metal belly pan. A belly pan will smooth and accelerate airflow going under the truck, which will increase fuel economy and high speed stability. This is especially crucial for big 4WD trucks like the Ram, whose axles and transfer case make the chassis about as smooth as the moon.
Fabricate or pay someone to fabricate a fiberglass Aero Tonneau cover. Aero Tonneau covers are completely different animals from Flat Tonneau covers, which are merely flat sheets that cover the bed. An Aero Tonneau cover tapers down from the top of the truck's cab to the top of the tailgate, and essentially turns the bed into one massive fast-back roofline. As of 2011, true Aero Tonneaus don't exist for this application, so you will have to fabricate your own, but in doing so you'll end up with a truck that looks like no one else's.
Get rid of the dual tires and install a set of 20-inch rims no wider than eight inches, and go with a set of lower-profile, all-season Touring tires. A set of 205/60-R20 tires will work best for this application; they're only about 0.5 inch smaller in overall diameter, so your engine rpm will remain about the same. You'll rarely hear this from anyone with a clue, but big rims and skinny, low profile tires are dynamically perfect for this application. This package will greatly reduce rolling resistance over the stock 245/70-R17 (10-inch wide) tires, but will do so at a slight penalty to ride quality and traction.
