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 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
Some feedback: I tried to convert m/s in km/h and it wouldn't works, finally found how to do it, you need to specify `hour` in full letters
I assumed it was something like an etherpad that you could edit collaboratively but apparently not