This concentric-hose setup is Ford's way of saying "eff you" to the customer - basically when you fill the tank the fuel flows through the inner hose, and the air it displaces in the tank gets pushed out through the gap between the inner and outer hose - if you have a gas-powered truck this means it takes you forever to fill up your tanks, and if you have a diesel like me it leaves you stuck looking for a car-diesel pump and then trying to get to it with a big trailer. There is a solution to the problem, it involves removing the rollover valve from the top of the tank and installing a barb-hose fitting in its place, then running a large hose from said fitting to the filler neck where you drill a hole and weld/braze on another fitting - now you can remove the inner filler hose and use the full diameter of the outer filler hose to pump fuel in, and the air will get vented through the top of the tank and that hose that you added. Basically you're creating an external vent for the tank (like many older Chevy and Dodge trucks have), it still vents inside the metal filler neck like factory, only you can pump fuel in faster and in case of a diesel truck you can now use the big-truck pumps at any truck stop. It should be mentioned that to keep things legal you probably need to add a small atmosphere vent somewhere, I've seen people t-ee that off the big vent hose.