A 20MPG (11.76 l/100km) car is twice as efficient as a 10MPG (23.52 l/100km) car though?

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.

> A 20 MPG car is not twice as efficient as a 10 MPG car.

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.

I might be hung up on the word "illusion". The only difference between miles per gallon and gallons per mile is that the former increases and the latter decreases with efficiency. They are still saying the exact same thing.

"They are still saying the exact same thing."

They are but now I prefer .05 GPM to 20 MPG.

I do as well.

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.

In fairness, 0 being cold and 100 being hot is pretty intuitive. I would be curious to know how people raised on Celsius feel about Fahrenheit after having lived with it long-term.