Closest I can give you is 2011 preproduction numbers I saw when testing two years ago, and they're to be taken with a grain of salt, lol.
Running unloaded from here to Death Valley in an unloaded CCLB 4x4, I averaged 18 mpg while running between 65 and 80-ish mph. During that time, I was writing and testing control code for the "hydrocarbon injector" in the aftertreatment system. It is basically a single fuel injector that sprays fuel upstream of the DOC to create heat in the exhaust to burn off particulates. Without that running all the time, I would've been closer to 20 mpg.