Winter conditions don't allow the fuel to vaporize much then the winterized fuel doesn't have as many BTU's per pound either. Add in some warm-up idling time and it all means the same thing, less mpg.
I plug in the block heater, start & go, a piece of cardboard between the aftercooler & radiator and drive the speed limit & no faster than 60 mph to maintain some resemblance of fuel economy.
I have same year, 4wd, 4"magnaflow exhaust, k n induction, edge juice with attitude, auxiliary fp. ,are dcu, kargomaster rack, bedslide. 24mpg highway, 17 -18 city if I drive reasonably light on throttle. On highway I found that with the edge on level 3 you dont really see a significant drop in mpg(according to the computer) even under fairly hard acceleration. I think the edge sets up conditions for more efficient burn and of course as you accelerate the turbo works much more efficiently so mpg barely drops off. Startin to love my cummins! The sound of the turbo winding makes me feel reel good. lol