Any mods? and no offense but, is your MPG calculation accurate?
Most of my XB's have been modded (muffler, K&N, programming) and I ride poorly, but MPG is usually way above 40. In fact I just fought a DAY of 40mph headwinds at 80+MPH (SUCKED) and got 43 mpg for those 2 tanks on my STT. I'll get 51 if I don't ride like an A-hole, but thats not often
Edit: Something as simple as an intake leak would do that. It's easy to test. The manual can be found at Buellmods.com.
You can easily check the AFV in the ECM using the free ECMDroid program (google play store). It needs a Bluetooth dongle to wirelessly connect to the bike. I like mine from Buelltooth.com.