i've done this test a few times now.
can't remember when the low fuel lights comes on exactly, but i want to say 30-40 miles left?
then after 18 miles left it goes to ---
i fill up then and put in about 18-18.5 gallons in. so that would be about 2 gallons left @ 14mpg city, or ~30 city miles left after the miles to empty goes to ---