I had the dealer rejet the carbs (since it had been so long since I done something like that) when they installed the 'off road silencers'. AI removed and I have the Leather Lyke bags which are pretty good size and drag. In suburban hilly country with some stop and go I get 46 to 48 MPG. The dealer thought the mileage would get worse but it has stayed the same or gotten a mile or so better. I've had the dealer tunes the carbs becuase of loud popping in 5th gear SOMETIMES. That drives me nuts-I can't tell when it's going to pop.
ANyway, I think everyone is right, you have something that isn't working correctly.
