running with the tailgate in the down position or removed is a bad idea, you will get worse mileage
tests have been run by different testers, including MythBusters
i believe a good tune up, clean air filter, & a carburetor or throttle body cleaning will almost give you the best mileage possible.
Remember, winter gas formulas burn more fuel than summer gas
2009 Ram 1500 SLT QC, HEMI , 3.55 axle, Bushwacker flares, soft tonneau, Line_X, DEBO Step, Mag-Hytec, Weathertech mats
2016 Ram 3.0 diesel 3.55 w/LSD, G2 tonneau, Line-X, Weathertech mats, Mag-Hytec differential cover, Nfab bars, CB radio, Autonet wi-fi router, EGR bug deflector