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If you completely and cleanly sever the fuel line between the gas tank and the fuel pump and let gravity do its job, you will drain the tank at a rate of about 1/3 gal/min. his is assuming (1) you have a gas/diesel mix (see my previous posts on this topic) and (2) your pickup tube is in an unknown (but soon to be known) condition, and I'm sure sever other unhitherto unknown variables. Once you get to draining about 24 gal off, the drain rate drops significantly to a point where you are glad you have an older style M37 with a drain plug on the bottom

Yeah....takes a while, don't it?

I once had the "privilege" of draining the 25-gallon tank of my Cadillac limo by siphoning the gas through a 1/16" ID plastic tube (the only thing that would fit through the holes in the filler-pipe baffle) when the pickup line broke off inside the tank. No drain plug on the bottom, and no way to jack the car up enough to drop a 3/4-full tank without some sort of lowering device. I got the job done, but it was an all day job filliing 5-gallon Jerry cans and then dumping them into a 35-gallon drum.