Hahaha, filling up and subtracting from the tank capacity won't tell you when the pump will suck dry, lol... Inquiring minds like to know... Will you stall with a 1/2 gal, 3/4 gal or how much left in tank? Plus you're starting with inaccurate data the other way, i.e., have you actually filled 17 gals into the tank?

Do it right... Run out of gas and remove all the mystery!