> And I hate it. miles-per-gallon is much easier to understand I think.
It's just something you're used to.
> I'd rather know how far a tank will get me than how much fuel I'll use to travel exactly 100km
Well, there you go, you're comparing apples to oranges. Unless your tank is exactly one gallon, you've made a conversion when you compute how far a tank will get you. Well, we do the same thing here. I want to travel 500 km? I'll multiply my L/100km by 5 and get how much gas I need. This tells me how many times I'll have to refuel, etc.