Those mpg's seem realistic. there will always be btu content differences between winter and summer formulations. strictly town driving (I like to accelerate) yields 15 mpg and I have attained 20-21 mpg twice on easy highway rides on out of state gas (CA reformulated gas sucks).

If I try to take it easy on the road without optimal circumstances, 18-19 is pretty typical on my 6 speed OR. I'm going to rethink those 285's and lift if means never seeing the high side of 16 mpg. My experience with my jeep wrangler was that the lift killed mpg as much as the tire size increase. I suspect the same holds true for most trucks.