I think that is BS. we have an '07 F-550 that weighs 14,800lbs going down the road every day. .488 gears in the rear end and we average about 15 mpg. We also have an '08 F-350. Never gets loaded, gets driven like it's stolen and averages 13.2 mpg. But, that isn't why you buy a pickup. If you're looking for fuel economy, you buy a car. In the defense of the 6.4, it'll pass everything on the road but a gas station.