Uh . . . . that's horrible mileage. Mine is 43.4mpg (indicated), and 41.8mpg (calculated). My normal top speed is 75mph (+/-), with occasional excursions into triple digits. Most of the time I'm anywhere from 45-65mph. If I keep it a steady 60mph, I will get 60mpg. The only time that I've seen your mileage is when I'm doing 70mph into a 25mph+ headwind. I'm 300lbs, and usually don't have the boxes mounted. I'm always in "S". It's a '17.
So, if you're usually hauling ass, all loaded up into a headwind . . . . you're mileage is normal. Otherwise it's way too low.
EDIT: the above information is all true, but as Stefan reminded me . . . . I don't ride for mileage. I ride as needed to deal with traffic. If I was more patient, I am sure I could get 4/5 additional mpgs. As previously stated, a bike this big with the power that it has . . . . getting the mileage it does, is greatness.