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I've included some captioned photos in the attached file.

One word of caution: If you pursue the gas cap guard plan please do not use a steel can.

In my case the leakage was limited to those times I was driving after a 'fill-up'. If the leakage persists when the fuel level is at or below 3/4 full, then start looking again for a leak elsewhere.

dp

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My gas spillage is right after filling up, so I do not think there is a leak anyplace else. Here is a pic of my first attempt. The coffee pad container is some aluminium foil and plastic combination. I could see that the filler neck was wet from fuel that came out the vent and was directed away from the fender by the can.
Something with a hose like JWL suggests would be the next level...
Also a pic here of the rubber grommet I added to the hubcap for quiet running.

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I am still getting some gas spilling out the gas tank cap. I am at About 7/8ths of a tank, and I do take some turns at 3, 4 or maximal 5 m/ss of lateral acceleration. That is 0.3 to 0.5 G in a turn. Not sports car territory, but for a packard I would call it "moving" or hustling
When I got to work, I could see that the coffee container was holding the fuel from the fender, but allowing it to wet the filler tube...
Maybe it's time for new shocks to reduce body roll? That would be the first step before looking at swaybars.
Another Option would be to raise the filler vent tube into a "shephards crook" that goes higher into the trunk and then vents down towards the Ground. Also a sizable mod that nobody ever seemed to Need.
I guess it means my engine and suspension are up to snuff! Happy packarding.

Here is what I am thinking: Put a T into the rubber hose that is the current vent between the tank and the filler neck. From the T, run a hose or pipe up to the top of the trunk. From one of the bolts that hold the tail fins on, I can hang either a gas tank vent valve, or a roll over valve. Then, from the other side of the valve, run another line out of the trunk and someplace down low to vent any gas outside of the trunk. This puts the valve up high so no gas can spill out and I retain a gas tank vent. I would then seal the vent in the gas cap. With a roll over valve, I have the added safety of not spilling all the gas out in the unlikely event of a roll over.
Basically, adding 1960's or 1970's technology to the gas tank. On the HAMB there seems to be quite a few people that have done stuff like this to hotrods.

Have you been able to identify where the gas is coming out of the cap? If the vent hole then that new vent might solve the issue but if from around the edge sealing surface between the cap and tank opening I doubt changing the cap for an unvented one will do much good unless the new cap has a better or different seal.

Any kind of experiment you could do to temporarily seal the cap and tank edge area differently to rule that out as the source. Maybe a thin but wider rubber or cork ring or something along those lines would seal tighter around the edge or else wrap something around the filler pipe and see if you can identify where the leak is originating.

This morning I folded a paper towel into about a 1 inch square and taped it over the vent hole in the gas cap. I also taped a paper towel around the circumference of the cap. Just after going through my first right hand turn, I pulled over and looked at the paper towels. The square over the vent was wet with gas, and the paper towel around the circumference of the cap was wet on the lower half. From 3 o'clock to 9 o'clock it was wet. The top half was dry.
So I think it is the vent that is letting the gas out. I have two caps that both have this behaviour. I tried making another paper gasket (paper gasket material) on top of the gasket on the old cap and gas still came out. I suppose I could try a cork or rubber gasket to seal the cap to tank interface. I'll also double check the filler neck surface for being square.
Or, I could seal the vent on one cap, and install it right before taking a right hand turn. I don't want to be driving too long with no vent and suck in the gasket on my tank sender or create some other problem. Then, if nothing comes out, I can pinpoint the source of gas to the vent.

I still think a vent hose described in my earlier post is a good solution. JWL

Still trying here and seeping some gas. On my old cap, which I am pretty sure is an old stant G37, I made a rubber gasket to go over the paper gasket on the cap. Driving to work this morning and using my paper towels and coffee container, it looked like the leak was not coming from the vent, but the sealing circumference of the cap.
With the added thickness of the rubber gasket, I can't rotate the cap all the way around to the stop.
Do most people tighten the gas cap all the way until it sits on the stops, or just until it is snug?
Maybe I'm doing something simple and wrong

Have you tried a different cap? If it's leaking, then the only thing it can be is that it's not sealing so maybe the cap itself is defective.

I've tried 2 caps. Both are leaking. I have another coming from dwight Heinmueller in 2 weeks and will try that. Here are pics of the caps and rubber gasket I tried today. The old cap does leak more.

In other news, my gas and temp gauges are not doing well with that drok voltage reducer. I am going back to original packard parts there.

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