65 Mustang Gas Cap Gasket

I don't think it is supposed to be sealed like that. New cars have an evaporative emissions system that keeps the gas vapors from getting too hot and expanding in the tank. These old cars did not have that and most likely just didn't seal well at all. How bad is it? I get a little "pff" on hot days when I open the cap on my truck.

Is this a new gas cap? Make sure the gas cap is properly vented. I replaced my gas cap and a lot of the caps being sold were non vented. I managed to find one that was vented. You can add a small line coming off the filler neck behind the gas capand run it to the outside of the car, keeping it away from moving parts and heat sources.

Did you have this problem with your old cap?

Look for a small hole or something on the inside of the cap. I bought the lockable key cap because that was the only one I could find that said vented in the description.

My cap had a little valve looking thing on it, and I blew through it to make sure it was vented.

If you have a well enough ventilated area and the proper containers, you can drain some gas out of the tank by either the drain plug or the fuel line into a container, look in the trunk, and see if the tank is trying to collapse into itself.

If you look at this picture, the lower right close up has the vent in the center of the cap.