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).