The UK may be the most confusing; fuel is sold in litres, but fuel efficiency is expressed in MPG, and furthermore the gallons aren't the same as US gallons. I guess at least the miles are the same!
Only since the 1958 International Yard and Pound Agreement tho. Before then the US used what is now known as the Survey Mile, which is why the survey mile exists (and survived until this year).
(Quite normally for my age in the UK I think, I'm familiar with both metric & Imperial measurements, but generally fairly bad at converting. Except I know 568ml = 1 (UK! Not US!) pint - for which I can thank my alma mater Imperial and its student bars: Metric, and FiveSixEight. I could probably guess effectively at lbs and kg from butter/flour. Of course I know 2.54cm = 1". A yard is 'a bit' less than 1m. It's the bigger ones that seem more obscure/are harder to work out from familiarity I suppose.)
1. I think with liters, people typically reverse the relationship so it's liters/100km. Which is a much more intuitive unit.
2. If you're buying gas in liters, I think it'd be a lot easier to switch over to using liters for efficiency. You may not be able to compare easily to other vehicles, but you'd be able to estimate your personal fuel more easily.
I think it's the other way around. Distance per quantity of fuel is the intuitive measurement that humans understand and can relate directly to how much fuel they purchase. It could be argued that it is less intuitive when comparing two cars, however. Although better MPG is still strictly better, which is about the level of detail most non-nerds care about.
At no point was it ever illegal do display the old units. There were no martyrs; there were only idiots.