Gasoline Evaporation of 3/4 gallon per week ???????

Brad777 wrote:
I fill it at home with a small gas jug. When the fuel tank is full it will not take anymore fuel. I then cap it off and wait one to three weeks. Then when I fill up the tank again I measure how much fuel it took to full.

Gas expands and contracts with hot and cold. If you're filling it to the brim you are leaving no room for expansion. As it expands gas will go out the vent tube and then it contracts and leaves you room to put more gas in so it can expand out the tube again.