[manholio]

> If I need to travel 260km at 20km/litre I need to but 260/20 litres or about 13 litres.

> With the “metric” way you need to do two calculations.

That's really an absurd way of looking at it, because one of those calculations is a division or multiplication by 100, and 100km happens to be a typical distance someone might be interested into finding fuel efficiency.

So the answer to me is an immediate mental computation, 2.6 x 5l = 13, you never ever need to compute consumption per km, express things in ml etc.

Your own "easy" examples look to me very intuitive and convoluted, so I guess the systems are 100% comparable, the only difference is the power of habit.