[lovehamr]

In addition to what Ron says I believe that where (height) the vent tube connects to the filler neck can have a huge effect on how it acts, the higher the better. In effect, if the fuel that you are pushing into the tank is trying to enter the vent tube as well then you experience a back flow as though there were no vent at all. If however the vent opens to an area above where the fuel is being introduced then the displaced air has a place to go and you're not creating your own plug in the system.