Gas overflow mystery ?!?!?!

[Glennster]

I've got a California built/ready 1970 Challenger. We replaced the gas tank last summer. I also use the plastic locking gas cap that hides behind the high dollar chrome snap shut gas cap cover.
 Here's what happened, gas leaked out of the cap, ran down the side and bubbled the paint. I drove the car two weeks ago and it looked fine, I put a little gas in it that day, but not too much (I think?).
  I've been smelling gas in the garage, not every day, just sometimes. The Chally is parked so I can't see the gas cap. Judging from the way the paint is bubbled, it looks like there was a steady trickle of gas that caused the bubbles. It's just a 1/2'' wide straight line down from the gas cap.
  I did find that the cap was not on right (stupid me), so it did not seal properly.
 BUT, why did the gas come out, I used some SEA FOAM to winterize the gas, I think I also used some STA-BIL. Could a mix of the two cause a reaction?
 The temperature in my garage ranges from 35 to 45 in the last few weeks....
 I'd sure like to know what I did wrong before I get the paint fixed.
Once again, I am in need of some help. PLEASE get me fixed, I'm big time confused on this one.
THANK YOU for any assistance!

[Alaskan_TA]

It could be thermal expansion.

If you fill the tank while the outside air is cool, then park it in a warmer garage, the fluid will expand as the temperature rises.

[MEK-Dangerfield]

The same thing happened to me once.     Now when I fill it up, I drive it at least 10 miles so I know nothing will leak out if it warms up considerably.

[LAA66]

 Which fuel tank or sender did you buy? It sounds like there is a problem with the venting of the tank. With no tank relief, as the fuel expands (even at cooler temperatures 35 up to say 55) the fuel gets forced up the filler tube. If you use a non vented cap also it may be dangerous especially opening it under pressure. 

[Glennster]

I had a buddy buy and install the tank, I'll get with him and see what he remembers.
It does seem to me that gas must have somehow forced it's way up the neck.
Thanks for the input!

[resq302]

We have this problem with our el camino.  We've come to just not fill it up all the way or drive a couple gallons of it off before we park it.  We also find it is worse when it is really humid out or it gets hot really fast outside.  Someone also mentioned that it has to do with the forumlation of todays newer gas.

[duodec]

Not sure about '70s  but my '71 ECS tank has an internal expansion tank on the inside top that is supposed to handle the temperature variations; it had a metering port inside that would allow gas to "squeeze in" against air pressure inside it (and had a rubber plug in the top of the main tank; I think that might have been a blow-off to prevent the tank from rupturing, but I'm not certain).

When I initially looked for a replacement tank, none of the repros had the internal expansion tank.  I don't know if thats the case now, or if your new tank also doesn't have one, but that could have been at the root of the problem, if so.  I'm holding off on buying a replacement tank due to other priorities but would love to get one that is totally correct in that aspect.

I lived with and drove my Challenger in Las Vegas.  I filled it up all the time on days when the temperature swing could be 40-45 degrees and never had gas escaping from the filler.