if motor was held on a dyno at a steady XXXX rpm, under load... the bsfc at that particular rpm, represents the fuel used/necessary to do so.
the lowest bsfc shown on the actual graph line, is still pig rich. (this is average graph of 30+ motors i had data for. some are quite a bit higher, some quite a bit lower of course).
as motor performance is optimized, a lesser safe bsfc is usually shot for on a dyno.
this graph does not have anything to do with "gas mileage" (ie. cuts per gallon).
usually a bsfc graph looks like an oyster. will post one up of this type.