Right so in that scenario that measurment makes sense but for the average person in their day to day life it's much less useful. And my car doesn't have a fancy computer that tells me the efficiency. Or measure in accurate increments how much fuel is in the tank. So if I wanted to know how efficient my engine is, the best way to do it would be start on empty, add a liter of fuel, and see how far I could drive it, which is probably why and how that measurement became commonplace. Because cars have been around alot longer than computers.