The metric system is a really good system but I don't know what retard decided to do the mileage like that.
The distance driven divided by the amount of fuel used is logical. I like Miles per Gallon and would actually prefer if the metric system did Kilometers per Litre.
The amount of fuel divided by 100km is really stupid but I've force myself to get used to it. We buy our fuel in Litres, drive on roads with metric speed limits and drive cars measuring distance in kilometers.