My favorite demonstration of this is that increasing your mileage from 10mpg to 12mpg, from 20mpg to 30mpg, and from 60mpg to 1,000mpg all save the same amount of gas.
So moving from 10-12mpg saves 1.67 gallons per hundred miles. Moving from 20-30mpg saves the same. At 60mpg, you're only using 1.67 gallons of gas, so there's no improvement to MPG that will save more than that. Nearly all the gains to actual fuel consumed will come from increasing the worst offenders rather than e.g. trading in Corollas for Pruises.
Also a good demonstration why the CAFE standards and other government fleet minimums are actually pretty effective at lowering fuel demand.
