Yeah, it's the Cummins 6.7-liter engine.
As far as what I did differently, it was pretty much all driving style. A big part of it was taking a different route to work. I used to take the interstate at 60 mph. Now, I take a nearly-parallel side road (4 lane divided but with many red lights). Max speed is 55 mph, but most of the time I'm at 45 mph or lower. It's pretty much the same distance (40 miles) and it takes me an extra ~15 minutes. It's taken a while, but I've gotten pretty good at timing the lights, as well as learning to do a little pulse and glide. You'd be suprised how far I can coast with my big truck
As far as the setup, the truck is more or less stock as far as aero-mods go. The only thing I did was taking my wipers off when it wasn't raining, and running with only 1 wiper when it was. I added a homemade bed topper and a partial grill block this weekend, so we'll see how those work out. I have made some engine/powertrain mods as well. I'd rather not get into too many details on those as they may or may not be considered legal by the EPA.