Expressing the savings resulting from increased mpg in terms of $/gl is inherently inaccurate. You can’t figure out how much you’re saving per gallon because you are not saving anything PER GALLON. You CAN figure out how much yuou are saving PER MILE. If adjusting one’s driving style increased mileage from 20 mpg to 25 mpg, you would be saving $.04 per mile at $4 per gl.