Slow down there cowboy! I wasn't saying you WERE going to get 5-6 mpg. I was only saying IF that was the numbers, I'd think on it. I've owned big block gas engines (460 Ford with ZF5, and 454 with auto [700R4?]) and the fuel economy varied wildly. The 460 gave me low teens, and the 454 was in the single digits.
I'm not slandering the 500, as I've never owned one. I know it is a great motor, I just don't know what mileage you'd see in a big 4x4 truck. It seems like you DO know what to expect, so please enlighten me.