I completely agree with the g/100 mi. It instantly removes that complicated translation of how much money is saved (or spent, in total) on a tank (if you're money-motivated).
So, what I've noticed about L/100 km is that the single-digit + one decimal place is finicky in instant FE displays. How about dL/100 km and move the decimal over one? I suppose it's easy enough to do mentally. Plus, the perception means little, having been exposed to the old "English" system forever.
RH77