I think the more important difference than miles or km is which is the denominator and which is the numerator. The rest is just some multiplication factors. As

I think the more important difference than miles or km is which is the denominator and which is the numerator. The rest is just some multiplication factors. As another poster pointed out, using [fuel]/[distance] (like l/100km) means I can easily estimate how much gas I need given my decision on where I'm going. Using [distance]/[fuel] (like miles per gallon) is only useful in a situation where I've filled my tank up and ask "Now where could I get to with all this fuel?". I don't think that happens very often. I suppose it might also be useful in a situation where you're low on fuel and asking "oh Jesus, can I make it to a gas station?", but most cars now have a "[distance] to empty" display on the dash anyway.