Just got a new bike and figured I'd post up. Its an '09 Buell XB12XT. I bought it in Boston and rode home to DC. Averaged 49.7mpg for the trip. That is with the three oem hardcases attached and an oversized windscreen. I generally cruised ~75-80mph, but got stuck in traffic in NYC for ~35-40 minutes and did a couple of hours of backroads in western Mass going to/from a friend's place on the way back. Pretty respectable for a stock 1200cc bike with luggage I think. As usual I generally accelerated hard away from toll booths and entry ramps and of course couldn't resist busting out the odd wheelie when no one was looking
FWIW a lot of motorcycles are less efficient at lower rpms. Gearing too tall will sometimes hurt mileage (and almost always hurt the fun factor). Buell says the XB12's motor is most efficient at 4200rpm as a general matter (of it'll vary with load etc). With stock gearing that works out to be about 85-90mph! My TL1000S also got no better mpg with a 18T front over a 17T. The wife's monster is -1 in the front and +3 (I think, not positive what stock was) on the back (so geared much lower than stock) and still does 58-61mpg all the time. It likes to spin up a bit.