Originally Posted by Eagle 33
but what is it? what does it actually allow?
From my understanding, the breather tube in the filler neck allows air to go out of the tank as the fuel fills. With the end of the breather tube longer, the air quits coming up the tube when the fuel covers the end of it. This makes putting more fuel in more time consuming because the filler neck then fills with fuel clicking off the pump nozzle. Because diesel foams, it takes a bit for the foam to subside allowing more fuel into the tank. Many people don't want to wait for this pump-the-neck-full-wait-for-the-foam-to-go-down-put-more-fuel-in process, driving away with less fuel in the tank.
By shortening the tube, it allows more fuel to flow into the tank before the end of the tube gets covered. This results in having more fuel in the tank when you drive away.
That's how I figure it - Some one else may come along to confirm or correct that theory.