And they determined this how? The overhead is wrong. Unless they are filling to the top of the tube, letting the air bleed out of the tank, and hand calculating, I don't believe it. My 5.9 gets 16 empty mixed highway/city. It gets 13-14 pulling my gooseneck car hauler, and about 11-12 pulling a 32' 5th wheel with a big profile front end...
Unless they drive 50mph and never leave Kansas.