well, it's not the best thing to do, i was just wondering if you ever accidentally had.
it's possible the guage says empty, and there is still 2 or 3 gallons left.
i've seen this many many times, being able to drive for 45 mins on empty.
but, for what it's worth, if you run it down to the empty mark, and then take it off and flush the tank out, then when you fill it you should know exactly how much it holds.