If you wanted to upgrade to an even better car, what mileage improvement do you need to again save X amount of gasoline?

You need infinite mileage; as Dave pointed out, you’ve already saved 50% of the gas, so to save that amount again, you have to use no gas whatsoever.

You ask for some help explaining how to put into words the concept “You save $100 per 1000 miles when you upgrade to a 40 mpg car from a 20 mpg car, but you save twice as much ($200) when you upgrade to a 20 mpg car from a 10 mpg car!” Here goes:

Doubling your MPG will cut your fuel costs in half. This means that the more you spend on fuel now, the more you will save by increasing your MPG.

If you are spending $400 a month in gas, doubling your MPG will cut it to $200, a savings of $200. Obviously, another doubling cannot cut it by another $200, because then you would be paying no money at all (and another doubling would mean you earn $200 a month?)

So, the biggest bang for your buck is in improving the MPG ratings of extremely fuel inefficient cars. As a matter of public policy, the single biggest change we could make would be to prevent car companies from considering SUVs to be trucks for the purposes of fleet fuel economy calculations.

If SUVs had to be included in fleet fuel economy calculations (CAFE standards require a fleetwide average economy of 28 MPG, I believe, but trucks are not included in that calculation), the Big 3 would have to make smaller, lighter, more efficient SUVs the norm, which would, as Geekdad says, make a huge difference in our national petroleum consumption.