I've got to say that the 'per tank' designation doesn't seem to mean too much, since tanks are different sizes and people fill them to different levels. Measuring per gallon makes more sense. If you're really trying to minimize stops at a filling station then you'd count the number of full gas cans you carry with you, or something like that. Either way, it's somewhat arbitrary. But still, congrats on the 700+ mpg tank, because whatever tweaks you're using, it does mean you're getting very good mpg, and that's an accomplishment as far as I'm concerned.