“The difference in the mpg ratings doesn’t reflect the actual difference in fuel consumption the way that gallons/100 miles does, pure and simple. (...) the EPA should promote this system too. Sure, it’s trivial to calculate the difference, but with the current emphasis that car manufacturers, government agencies and consumers are placing on fuel economy, we need to change our way of thinking from high mileage to low consumption”

-Popular Mechanics

In Mexico it’s a known strategy of marketing to use km/l rather than l/100km to make a car appear more efficient than it actually is.