I'm sitting on 14.5 around town/city at the moment. Highway, as mentioned, 17....maybe 17.5 if lucky but I'm doing 70mph.
I've hand calculated a couple times but have always found the computer to be dead on. When it says how many gallons I've used the pump even cuts off within two tenths. I'm easily entertained sometimes haha.
I used to run around on a 80hp tune and sometimes the 105hp tune but typically keep it at 40hp now. I turned it up the other day and still had the problem but still a fun. As my brother put it after he drove it with the programmer turned up a bit "there's just something about a fast truck". Especially if you do 4wd but you have to be careful with that.