I've avoided adding mpg because the UK and US disagree on what a gallon is and mpUKg/mpUSg is not very nice to look at. You can write "miles per gallon" or "miles per US gallon" because per is the same as the "/" operator, and you can sort of do "L/100km" but you can't convert to it.

I'll have to think some more about this one.

I think I see where you’re going. I’d be comfortable with “litres per hundred kilometres” if that’s possible and especially if there was a “did you mean” suggestion prompt