The oft-quoted rule of thumb on here seems to be about 1 litre per nautical mile, which is about 4.4 statute miles per US gallon.
Matth suggested 0.6 L/nmi in the second post — for a similar sized boat with a recent engine. That's 7.3 mpg (statute, US).
I can't believe a figure of 14 mpg (ditto). That's 0.3 L/nmi, which is half what the lowest figures seem to be on here. Of course there will be quite a bit of variation between boats, engines, payload, conditions and people helming, but 7 mpg isn't 'about 14mpg'.
I think he's made a mistake measuring his fuel consumption, or accidentally put a 1 in front of it.
I'd plan for 1 L/nmi, 5½ nautical mpg imperial (or 4½ nautical mpg US), and then be pleasantly surprised if your consumption is better than that. It's much better that way than planning for 14 (or, being more conservative, 10) and being disappointed somewhere out at sea