I just wanted to say that for gas economy, l/100km is a better way to do it. Gal/100mi is just as good. To see the point, let me ask a simple question. Which is a better improvement, to go from 10 mpg to 20 mpg or to go from 20 mpg to 40 mpg?
The first of course (or I wouldn’t have asked the question). If you get 10 mpg, you need 10 gallons to go 100 miles, while at 20 mpg, you need only 5 gallons. You cannot improve by another 5 gallons unless you discover perpetual motion. If you get 40 mpg, you need 2.5 gallons to go 100 miles, so the improvement is only half as much, not twice as much.