Art, I have an old 04 Cummins and regularly get 11 -12 L/100km, that's with a truck that probably weights over a 100 more than yours. You claim to get 16.8 L/100km with a 6000lb trailer at 65-85 mpg? One of my sons has a '13 Ecoguzzle, towing a 17' all aluminum enclosed trailer with 2 snowmobiles, we averaged between 22-28L/100km at no more than 70mph.