Interior extension after trimming it off.

Inside the tank after trimming off the extension.
Everyone knows how long it takes to completely fill the tanks on their OBS. Its not unusual for me to spend 20-30 minute to milk the last 4-5 gallons into each tank to get an accurate fuel mileage report. It's painful and embarrassing, especially when there's someone else behind you, waiting for the pump.
There's already a few mods out there like the fuel vent mod or the dagger mod, but I think those are for later OBS trucks with plastic tanks; mine are steel. I already trimmed the inner rubber filler hose about even with the outer filler hose.
Ive been putting off fwd gas tank maintenance, but I know I had a broken shower head pick up in the tank. I installed new silicone body mounts today on my 1994 IDI, which requires dropping the fwd tanks, so today was a good day to fix the shower head. After replacing the rubber, I looked inside the tank and saw the filler nipple extends into the tank by about 2.5 inches. The steel nipple extension is too thick to use a sheet metal shears on, but a Dremel tool with a cut off wheel did a good job. Cutting a steel fuel tank with an abrasive wheel makes sparks and can lead to explosions. I sopped out as much fuel as I could and used a shop vac to catch the sparks and draw fresh air into the tank while cutting. It wasn't OSHA approved, but I'm still alive.
Long story short, after reassembling everything, I stopped at my favorite gas station for a fill up. The fwd tank was bone-dry, so this is a rather accurate test. It auto-clicked-off at 20.4 gallons. I was able milk another 1.5 gallons in before the fuel started backing up the filler neck. So the fwd tank capacity is 21.9 gallons of anyone is curious. Either way, this will save me a lot of time at the pump and was definitely worth a cutoff wheel to accomplish, but be careful and don't try this on a gasoline tank, due to the explosion hazard.

 

i run the driver side tire up onto a 6X6 block.
fuel will go in as fast as the pump can supply it.
off the fuel truck, i can dump 15 gallons into the tanks in under 1 minute.

 

with out any tank mods??

 

only a 6X6 under the driver side rear wheel.

 

I never realized that, and I've owned quite a few 80/97 F-Series trucks. But if you ask me, all the tanks these trucks came with were much smaller than they should have been, and it's great you figured out a way to get a few more miles out of them. Just be careful though, the headspace is in the tank for a good reason- OK to fill it up into the filler neck if you're gonna use the fuel right away, otherwise it will have a tendency to leak out the vent. I say this from personal experience, not theory.

This thread reminds me of the early 2000's F-150 truck that had exactly the same fuel tank with either "tank size" option, but if you didn't pay for the "larger" tank, they gave you an extra-long filler neck hose that extended about halfway down into the tank.