I had the same problem when I first got my bike. After riding a few months, and I can't really explain it...if I keep the thing spinnin up around 4K I get 42-44 MPG. If I lug it ( man that sounds crazy, luggin at 3K lol) my mileage drops back to 20-30. I.e. the Bonnie likes to spin up pretty tight for the best fuel mileage or my Bonnie likes it. YMMV.
