[doikor]

> The numbers don’t work out too, “litres per 100km”. Why not millilitres per km (or litres per gigametre)

> I drive X miles (or X km), I then fill up with Y litres (or Y gallons)

This isn't what most use the liters per 100km for though. People just fill up whenever the light on the car tells them to. Mainly the number is used to compare cars when buying a new one. Some keep track of the actual consumption to notice if something is wrong with their car (just write down the current number on the odometer and how many liters you put in on some paper/app/whatever. you can do the actual calculations later)

At least this has been my experience here in Finland.