So being a very curious person I canâ€™t stand not knowing how things work. Thanks to the wonderful folks at Google, I can usually find a fairly quick answer. So when I got a burning curiosity to know how gas pumps shut off when the tank in my car is full (and my credit card is empty) I was surprised I couldnâ€™t find a quick answer.
Not being satisfied with my initial findings I moved on to more advanced search techniques (really I just kept trying different search terms in Google.) From there I came up with this article from wonderquest.com (you’ll have to page down past the article about sleeping birds.)
The short story on this is basically there is a small vacuum tube that runs down the spout and has an opening near the tip. The pumping gas produces suction on the vacuum tube and when gas gets high enough in the tank to cover the end of the tube it increases the vacuum on the other end of the tube which trips the shut-off.
Think about it this way: if you were sucking air through a straw, then someone put the other end of the straw into a cup of water you would notice that it became harder to suck. With any luck at all, you would stop sucking before you filled your lungs with water, just like the gas pump stops before it fills your shoes with gas.
For a better explanation, here is the article from wonderquest.com. Another interesting article from Husky describes the technology, as well as goes into more detail as to why it sometimes fails (as well as describing why their nozzles are the best, of course.)
All-in-all a very elegant, low tech solution requiring no electricity, sensors, floats, or any other fancy gadgets; one that has passed the test of time.