Well, I took it as low as I dared, which was about one needle's width above the line to E... It took 18.75g to fill up, or 626km... do we supposed there could be 3 gallons hiding in there still???
I've chased this 'hidden' fuel before and finally found it. Like Tom says, you could try for 700km or you could just consider that 3 gallons an emergency reserve that's good for 100kms.