The best (and oldest) trick to better gas mileage is actually FREE! Everytime the nose of your truck is facing downhill, bump the shifter into the 'N' position. Any time spent at idle RPM's is better than even high gear under load.
Just remember to drop it back into 'D' before doing the redline over rev.
A couple of those, and you won't mind buying more gas.
2012 F150 Harley Davidson Edition that Ford shoulda built! Tuxedo Black Metallic, 2.5 " BDS Front Leveling Kit, Readylift 3" Rear Blocks, TIS34 Monster Energy 20X9 Wheels, Richard (can't say D***) Cepek 35/10.50/20 FC-II Tires, Bushwacker Extend-A-Fender Flares (PTM), and Leer 180 Mid-Rise Cap (PTM), Full Tint, & Weather Tech Molded Floor Mats. More pics in my Garage.