In America we say the fuel economy of car as miles/gallon (miles per gallon). I know in some countries people use Liter/100 km (liter per 100 km). Some countries use km/Liter (km per liter).
Which format do Japanese people use, L/100 km or km/L?
That would be km/L (only).
Fuel efficiency is called 燃費 and "fuel efficient" or "fuel inefficient" are called 燃費がいい or 燃費が悪い.