I towed my 7x14 extra tall cargo trailer across canada. Truck used a fair bit of DEF going east with a tail wind in the prairies at 125km/h 75mph 20L/100km Northern Ontario was better as i was going slower. 110km/h or 65mph. 17.5L/100k
However, coming back west into the wind across the prairies. trying to do 125 as I wanted to get home. Fuel mileage got really bad. I had to slow down just to make it to the next major city to fill up. 23L/100km Def usage was brutal. I used over 3 jugs for the 3600km trip back.
Now I also have a 3500 6.6Duramax that has pulled this trailer. It does trips to the USA alot, it averages the same fuel mileage roughly 17.5 to 20L/100km. But uses far less DEF. My thoughts are the exhaust system on the 6.6 is much larger and designed for towing. So it is probably better at handling the exhaust for the amount of fuel burned. The 3.0L dmax, while great at towing, i think it's main purpose is commuting. If you're towing, bring lots of DEF.
