They do have the same ratio in top gear however the 8 speed is more efficient so there is mileage gained there. Also most of your MPG loss is in getting up to speed and passing where the 8 speed is advantageous as well. If I'm not mistaken I don't think the 6 speed trucks have the grille shutters either.
2013 Ram 1500 Lone Star Quad Cab 4X4. Bright White (PW7), 345ci Hemi V8, 8HP70, 3.21 gears (LSD rear). Mods list
2015 Durango SXT Southwest Heat RWD. White Knuckle (PW7), 220ci Pentastar V6, 845RFE, 3.45 gears (open). Huskyliners, Catch can. Thread