While the EPA does not rate fuel efficiency because of GVWR, the real problem is that the range of fuel economy varies greatly depending on driving conditions, load, driving habits,etc. Many different people will give you different answers, and they will all swear that they are right. Unloaded highway driving you can expect 20+mpg, driving below the speed limit. You will lose approx 10% fuel economy for every 10mph over that speed. Another factor to consider is that the new diesels use extra fuel to clean the Diesel Particulate Filter on the exhaust, providing a net loss of up to 3mpg per regen cycle, approx every 300 miles or tank full, depending on load and speed. Real world driving expect to see anywhere between 10mpg towing and 18mpg unloaded.