So recently I noticed a puddle of gas behind my car

It looks like it's leaking from somewhere higher up, and it's collecting in the heat shield above the muffler and dripping down.

I had the tank out a couple months ago and had it cleaned. I do not recall it leaking until now.

Anyone have a clue where it could be leaking from? I don't think it could be from the fuel lines since they are on the other side of the tank and not above the heat shield. To me, that just leaves a puncture in the tank, and the sending unit lock ring. With 1/2 tank, would fuel come in contact with the lock ring seal?

 

Was your tank rusty and crusty before it got cleaned? The leak could be starting in a totally different spot from where it's dripping down, so don't assume it's from straight above where you see it. If it's leaking from the lock ring, it would have to be from while you're driving and the gas being sloshed around, not after the car is sitting. It could also be coming from a fuel line connection at the sending unit. You need to get under it and see if you can find any wet spots on the tank. Take a mirror with you and see if you can also get a look at the top of the tank.
Also, is your tank well vented? I've had a tank leak at the sending unit from a bad venting gas cap. Pressure built up a lot, and it had to go somewhere. So it pushed past the o-ring. Good luck, and don't blow up!

 

Yes, the tank was rusty. I figured the leak wouldn't be from the sending unit. My first thought was that pressure was building up and forcing it out somewhere, so I pulled the cap and there was zero pressure. Tried this about 3 times now.

Say I find a pinhole in the tank, can this be patched if I run enough fuel out of it to be below the leak? If so, what would you recommend?

 

I've fixed the bottom of an old Vette tank before, and it held up well with no further leaks. It had a lot of pin holes in about a 3" radius. If yours is on the side, better yet. In my case, I obviously had to take the tank out. If you have a leak on the side where you can get to it, drain the gas low enough so it doesn't leech out the hole. Clean the area well with a cleaning solvent first, then scrape the rust off if you have chunks. Then sand it to get the rust all the way off. The coarser the sandpaper you use, the better. Get yourself some Epoxy, and fiberglass cloth. Mix a good sized batch of epoxy and smear it onto the tank covering the hole and beyond it. Cover the epoxy with a piece of fiberglass cloth and keep dabbing it (with a finger or popsickle stick) until it gets saturated. Then smear more Epoxy over the cloth and onto the tank. Basically you're fiberglassing, but the Epoxy is a glue where fiberglass resin really isn't. Unless you can work very fast, 5-minute Epoxy might set up a little too quickly to give you enough time.

 

hmm. im going to take a wild guess and say its a fuel line leaking. probably where its rubber. once theyre disturbed they tend to leak after being in spot for so many years. drop the tank (now that everything has been loosened) and replace whats leaking

 

All the rubber lines are new with stainless clamps.

 

easiest way to find out is take it off and look. you could also use a smoke machine and try to find the leak.

 

Well it got worse, I was leaking quite a bit. Pulled the heat shield off and immediately a small dimple in which rust formed became apparent, right in the sump of the tank. Luckily I only had maybe 3 gallons of gas in it, so I siphoned most of if out, then opened up the hole a little bit and let the rest drain through that. Then I cleaned the area and applied gas tank epoxy into and around the hole and let it cure for a couple hours. Seems to be holding...