In 2010 ,my company had a contract to produce a series of parts for a company 800 miles away. The parts required special handling, so I decided to use my 5.9 2006 Ram, 4WD with 3.73 and factory lift, as the delivery vehicle. This became an every two week trip, so naturally we began to play with ideas to increase mileage.
Hauling 800 to 1100# one way each trip, we were getting 16 to 17 mpg. Added 2" to the front air dam and added a mpg. Added 4" and got to almost 20 mpg. We ended up by adding full side skirts and rear diffuser and were able to get 22 to 23mpg. Oh yea, we averaged mid 70 mph, but then it was nearly all interstate.
(My 2012 5.7 4WD w/3.23 Overland gets 15 mph except when hauling at least a 1000#, then it gets 18 mph. Maybe I need to try the air dam mods again.)