They don't know. What they sense is liquid splashing against the metal spout of the nozzle, which gives a signal for the pump to shut off. This point in the evolution of gasoline dispensing is what the engineers and powers that be want us to think is a full tank. This is not the case. If one carefully controls the rate of gas flow by squeezing the handle gently, you will discover that a substantial volume of fuel can be added. In some autos, this can be another gallon or so. In trucks and vans, this added amount can be as much as four or five gallons. It is this deception by means of engineering that causes problems for the consumer. For example, if you want to calculate a precise figure for fuel milage, one must fill the tank COMPLETELY, so that one can see a static fuel level in the spout. You then drive a certain distance, fill the tank completely again, then do the math. Since different gas pumps dispense gas at different rates, the shut off point differs enough to render any calculations based on the auto shut off useless. Another problem often occurs in my line of work. I drive a taxicab. When I get the vehicle, it is "full", as gagued by the previous driver. When I turn it in at the end of the shift, I am to turn it in "full". Drivers never take the time or effort to fill the tank to the absolute limit....rather, they fill it until the auto shut off clicks a few times. The problem is that each pump fills at a different rate, and there can be a huge variation in the degree of fullness between one fill up and another. This can go way beyond a splash or two, and it's not uncommon for a taxi that is supposed to be full to be maybe seven eighths full. In this day and age of three dollar a gallon gasoline, this can add up, big time. Why is this this way? The reason is that the engineers that design both vehicles and gasoline pumps are motivated by arrogance and contempt. They do not believe that the average Joe can responsibly fill his vehicle safely. So, in keeping with the nannyist attitudes that have brought us mandatory seat belt laws, helmet laws, and Q Tips that tell us not to insert them in one's ear (PLEASE!!!!), we have gas pumps and gas tanks that do our thinking for us. And if you are average Joe or Jane Sheep consumer, this insidious bit of "knowing what's best for you" goes unnoticed. If , however, determining exact figures for milage and exact degrees of fullness are part and parcel of your life, you're just out of luck, bubba. If you're not outraged, you're not paying attention!