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Through the 1960s automobile manufacturers used a vented gas cap to allow air to enter the tank to replace fuel that was withdrawn by a mechanical fuel pump. That meant that a full fuel tank could leak with aggressive driving. And if the vehicle flipped over it would allow fuel to leak through the cap. Fuel tank systems became closed and tank vapors collected in a charcoal canister around the year 1970.

With your vehicle operator knowledge and technique are necessary when refueling. You probably have a 16 gallon tank on that Barracuda. When you stop and add fuel note your fuel level gauge. If it is at the 1/4 mark quick mental arithmetic will tell you that 16 - 4 = 12 gallons maximum to add. 1/4 tank would be 4 gallons.

Watch the gallons accumulate on the pump and listen to the sound of liquid as it enters the tank. As the tank fills the sound changes and with practice you will note a distinct pitch change as the tank approaches full. STOP before arriving at the full mark.

As liquid fuel enters the tank the displaced air has only one avenue of exit. It has to travel back up the filler tube. When you approach the full level the exiting air "spits back" the fuel to you. Stop filling before you get to that point.

If you wanted to make a permanent fix then you would have to retrofit a tank with a built in vent and route a vent hose high on the body. Again you would have a problem if the vehicle flipped over as this vent would leak fuel and that would create a dangerous situation.
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