Blimey!
I reckon on getting 75+ mpg (4.54 litre sized gallons) from my B in normal back-road running depending on mood, and never less than 60 from the A, often 70mpg. Not much motorway round here, 'tis true, but lots of up and and down the rev range. I'm used to my AMC twins being unreconstructed petrol junkies (less than 60 mpg from 500cc is pathetic), but I've always found the BSAs a bit better and more economical than most other things of the same size. That's a bog standard late B cruising at 50mph, and a bog standard s/arm Monobloc Flash with one tooth extra on the gearbox sprocket, cruising at 60-65 mph. Bog standard 85/95 Octane Euro-gas. 25 to the christian gallon, even with a heavy hand and a pair of mega-carbs, is ridiculous, and under 50 is thirsty even by the standards of those of us who consume wine in industrial quantities. But under 50mpg on a 3.8 litre gallon would start to sound a bit better!