A 20 MPG car is not twice as efficient as a 10 MPG car.
http://www.mpgillusion.com/p/what-is-mpg-illusion.html
L/100km has the length and volume units reversed, which allows comparisons.
The issue is going from 30 to 20MPG is not the same change as going from 20 to 10MPG, where as the difference between 15 and 10, and 10 and 5 l/100km is the same absolute change.
Why not?
The page you linked doesn't talk about "twice as efficient", it talks about an absolute number of gallons saved per distance travelled.
Also, assuming there was no typo, vegardx's point was that "kWh/h per 100km" is invalid. "kWh/h" is the same as "kW", and "kW per 100km" (power per distance) is not describing vehicle efficiency, it's non-sensible. "kWh per 100km" (energy per distance) is.
They are but now I prefer .05 GPM to 20 MPG.
Everyone in the US it seems to like bigger numbers. Just look at the argument some people use to why some people prefer Fahrenheit over Celsius.
... is what I would say, but Tesla seems to be the only one using the sensible Wh/m or Wh/km. All the German ones use the nonsensical kWh/h per 100km.