Can't say anything about an 80kph speed, but I tend to do 95kph and I am in the mountains and Alberta so pretty mixed driving.... get the hills in the mountians and the prairie winds in Alberta.
if I stay driving nice (95kph) I can get as low as 18L/100km (13MPG US/ 15.6MPG CDN) when I am in a strong headwind and try to do the speed limit in Alberta, that can drop down to 22L/100km (10.6MPG US / 12.8MPG CDN.
I did just tune my truck a bot better so I will see at the end of June what I get going to Alberta and back now. It sure improved my empty fuel milage.
Steve