Pumping gas- What an idiot!
My guess is that the filler assembly is either bent or just "misplaced". There is also a vent and a little hose running inside the trunk, to divert the gas fumes and may be gas (when overfilling) out side the car.
Take the inside lining of the trunk out (passenger side) and check. Maybe just the hose was falling off and then you will have the fumes in the trunk/car. The reason why it seems to be related to speed is that the inside flow of the air in the car is depending on the fan switch position, the temperature and other parameters. The "used" air in the car is leaving the car through some vents on the side behind the rear bumpers. And they are just above the filler cap.
So check what I proposed and if all is okay take a high pressure cleaner and spray the filler compartment (cap closed!!!!!!), behind the bumpers (passenger side) and the wheel housing.
It should work just fine. If parts are bent than you need to see whether there is a change to bend them back to how it was before the mishap.
On your problem, the gas camp does not have to fit completly in fact id it is missing altogether you should still have no fumes. try out the line that runs through the trunk...
i came back out and was so exited about the slurpie






