Ok. But if you have a 10 gallon tank, listing it as MPG allows you to know how far you can go between gas stations. Which is important
Not really. In the actual world, no one gives a fuck about your imaginary family that frequently makes 600 mile trips and would be mystified by measuring efficiency in gallons/10k miles, because we assume, they can't multiply by .06...
That was the scenario I was responding to though.
It's the same thing either way. The difference is that one is focusing on consumption (how much fuel used over a given distance), while the other is focusing on range (how far you can go with a given amount of fuel). It's not like people can't noodle out the math either direction, but in terms of usability, consumption is more useful when making your initial purchasing decision (which you kinda just do once). Range is more useful when actually operating the vehicle (which you do all the time).
MPG is a terrible measure of fuel efficiency once vehicles become reasonably efficient. As they become very efficient it becomes even less useful. I don't think it was intentionally designed that way, I think the US just has sh*tty, poorly thought out measurements in many areas and is resistant to change. Hence the idea that 34mpg is much worse than 40mpg when it's a marginal difference, and the idea that 12mpg is basically the same as 18 mpg when it's a huge difference.
I'm not really seeing how if we measured that as 2.5G/100M versus 2.94G/100M or 5.55G/100M versus 8.33G/100M it would help anything. People can either grasp ratios or they can't. And I'd honestly argue that it's easier for people to do ratios with bigger/whole numbers in their heads. As fuel efficiency increases, the fuel use per X miles gets smaller, making it appear as though the gains are smaller. One can also argue that from a marketing point of view "bigger is better" ties in better with using mpg rather than g/100m, so there's that too.
I suppose it's more a matter of what you're used to, but from an objective point of view, I don't see how one measurement inherently much better than the other, but for the reasons above, I'd tend to give it to the range view rather than consumption view.