When I was young I had a geared-up Norton N15 (750cc) that routinely got 55 MPG (US) on the highway and once got 60mpg (this is the same bike with a Matchless badge:
http://www.realclassic.co.uk/match03082900.html). Also had a Norton Commando (750cc) that was almost as good. I was not riding to save gas either.
I haven't bothered watching the mileage of later bikes because I don't take them out that often or on long trips.