My 2006 K3500 is the exact opposite. Empty never sees over 16, but does 11 average towing 25' gooseneck and hauling loads which I consider acceptable.

I am upgrading to Kennedy Diesel's tow/economy tune and plan to put it to the test hauling boats cross country. We'll see how the averages are, between a stock 7000 mile thrash, followed by the same 7,000 with the same boat/trailer combination.

More soon...