This is a graphic treatment of an issue mentioned yesterday -- the somewhat misleading nature of the miles per gallon statistic. If you use MPG as your main metric of fuel efficiency, then a change from a 20 MPG vehicle to a 30 MPG vehicle sounds like a smaller advance than does a switch from a 40 MPG vehicle to a 60 MPG vehicle. But if you assume a constant distance to be driven, the former switch reduces fuel consumption more.
Now, obviously, a 20 MPG reduction is still better than a 10 MPG reduction, all else being equal. But "all else," crucially, isn't equal. You get much more bang for your buck by improving performance at the low end.