Went to the gas station today with less than half a tank, filled it till it was coming out of the neck 6.6 gallons, drove 1 mile then came back and filled another 8.4 gallons till it came out of the neck. Still not sure the tank was completely full, but I’ve had this issue since I owned the car, the tank does not fill all the way for some reason. I assume it’s some venting issue? Gas gauge is also very inaccurate which I haven’t got around to fixing yet, not sure if that plays into the issue.

 

There is a breather tube on the side of the gas tank fill neck that connects to the gas tank right in the middle at the top of it's saddle. It's easily accessible from under the vehicle.

It's a metal line from the fill neck down to the point right where it attaches to the gas tank then it turns into a little rubber 90 degree fitting to connect to the tank. That piece is probably corroded on the inside.

It's held on with hose clamps to the fill neck tube and to the gas tank, you can take off that rubber fitting (wait until your tank is 3/4 to 1/2 empty) and visually inspect for anything clogging it up. I had to use a screwdriver to knock off some corrosion on the little nipple coming out of the tank (there's an internal fuel filter strainer baggy so just push the crusty **** into the tank if you have to). And I had to dig out a bunch of crap inside that rubber fitting.

When that thing gets clogged up the air can't vent back up out of the tank fast enough, gas pumps have a pretty high flow rate and the difference in air volume needs to be able to displace out of the tank. Otherwise gas just backs up into the fill neck and won't drain into the fuel tank.

Easy fix, took longer to get the rear of the car on jackstands than it did to fix the problem.

 

Here's a picture of the gas tank, it's this smaller line.