mpg on my 340

This has nothing to do with the tank venting. The tank vent..........a 1/4" tube.......could never ever flow enough air to vent a tank when filling. The fill issue is "right in the neck."

If this is a later car, might be the neck is protruding or damaged. It most likely is way too much pump pressure on modern nozzles. When I had my 67 running, there was only about 3 local stations I went to, normally, and there were at least 3 that I purposly stayed out of. One was so bad I don't know how anyone used it. It would puke over on my Ranger or the Olds. I complained to the typical check out girl, trying to get her to say something to the owner manager, and I could tell, "taking to the wall" made more sense.