And one that does 40 mpg will use half the fuel of one that does 20. Yeah, totally unintuitive.
Yes. But that's not the whole story. 40 will use half that 20 does. 20 will use half that 10 does. But 40mpg is 5.88 L/100km, 20 is 11.76 L/100km, 10 is 23.52 L/100km. So switching from a 10 mpg car to a 20 mpg car saves 11.76 L/100km, but switching from a 20 to a 40 mpg car only saves 5.88 L/100km. So that is why mpg is said to be exponential. This is not just me blowing smoke, see https://www.fueleconomy.gov/fe... and look at the section "Fuel Consumption Rate". Even the US department of Energy says Volume/Distance is a better representation (even though they stick to gallons and miles).
BTW, there are some advantages to L/100km. MPG is exponential, so as the numbers get higher, there is actually a diminishing amount of savings. So upgrading from a car that gets 14 mpg to one that gets 17 mpg is the same savings as going from one that gets 33 mpg to 50 mpg. With L/100km it is linear so the savings stays the same along the entire scale. All you need to remember when seeing it, is that the lower the number the better the fuel economy (and anything lower than 6L/100km is pretty good efficiency).