To travel 100 miles at each of those MPGs;

10MPG = 10 gallons of gas

12MPG = 8.33 gallons of gas

20mpg = 5 gallons of gas

30mpg = 3.33 gallons of gas

60mpg = 1.67 gallons of gas

1,000mpg = 0.1 gallons 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.

You can’t save more gasoline than you’re using. More:

The misleading, wasteful way we measure gas mileage, explained

https://www.vox.com/future-perfect/350382/gas-mileage-fuel-e...