Many years ago, my cousin, Baitfish2 bought a 50 gallon under the toolbox gas tank for his Chevy.
Unlike today, there were not a lot of options for those type of things. If it had a place for a gauge sender, we could not find one to fit.
Carbed engine of course, so we just put a "T" in the line between his stock tank and the 50 with an on-off valve he could reach inside the truck.
Normal procedure was to fill up both tanks, then when the stock tank got low, he would turn on the big tank and let it fill the stock one.
One day, headed to the hunting camp, he filled up the 50, at about $1.50 per gal. His stock tank was about half full, but the big one took most of his cash.
When he left the station, he turned on the valve to fill the stock tank so he would have a full tank to hunt on.
He never told what happened, but I suspect he either saw a good looking girl, or maybe a good song came on the radio.
All I know is, I was standing in the camp when I saw him come around the corner of the road.
Looked like there was something running out of the side of his truck.......
Sure enough, it was the last few drops of the 50. He had forgotten to turn off the valve, and it ran out the vent in the cap all the way to the camp.
He was not pleased. I got a good laugh out of it though.......