Zero your trip odometer and ride a few tanks for 75-100 miles. Divide the miles ridden by the amount of fuel put in, there is your average MPG. Now multiply that times 3.3, there is your estimated range until you are completely bone dry. I don't know why someone would ever try to run that low though. I really don't have sympathy for people that run out of gas. I'm looking for a gas station when I hit 100 miles or the light comes on, whichever comes first.
Just to be safe multiply it by 3 rather than 3.3. I don't think you can ever fill 3.3 gallons of gasoline in the tank mainly because the fuel pump takes up some space and there's some spacing issues where you wouldn't be able to fill the part of the tank higher than the bunghole.