[wtallis]

It seems to me that calling it "ink efficiency" is framing it in a way that ignores readability and legibility. The more common term for this is "font weight", which we already knew was independent of the pitch (size). It shouldn't be a surprise to anyone that the more generally readable typefaces are in the middle of the group.

[sax]

Yep, based on intuition it would definitely seems like fonts with similar font weights (assuming the same font size), would offer similar legibility. The real question is, given a choice of font and the requirement of printing a novel or HN front-page on a mobile device, how do you quantify the trade-offs between them, so that one can say, this saves 'X' amount of ink or power for the same legibility?