Gas filler and modern pumps
I would make sure your vent pipe is clear before doing anything else.
Personally, I'd be very leery of sticking a device in the filler neck. My Barracuda had one of those anti-siphon springs in the filler tube -- it slowed down the flow so bad that some pumps would shut off instantly, on the lowest flow setting. Took forever to fill the tank. I finally replaced the whole tube, since there was no way to remove it.
Now, I find a slight twist is enough to make it flow OK.