What kind of fuel mileage can you expect

My 2012 in my sig averages about 13-14 empty in the city. Its basically a dedicated tow vehicle and isn't a DD so I've never figured empty highway mileage. Towing 10k lbs I'm getting between 9.5 to 10.5 very consistently running 65-70 mph. Truck has 11,000 miles on it so it may get a little better with time but I doubt it. As low as those figures are it beats my 08 Powerstroke towing by 2-3 mpg. The power with the Cummins is truly unbelievable. It runs quiet and is just a fantastic tow vehicle.