I get 16-17 highway on Summer gas and drive 70-75 on the interstate. I'm also not typically hauling/towing, driving in pretty flat country, and in areas with reasonable traffic flow (ie, not a bunch of deceleration/acceleration cycles, stop and go traffic jams, etc). Still on stock tires, not much in the way of heavy add-ons (rock rails and front diff guard only).
Winter gas drops it to 14-15, strong headwinds do the same or worse. Having a bed full of firewood drops it a bit, but not all that much. I suppose the percentage add to the total weight of the truck isn't that much for wood since I don't stack it much above the bed rails.