BB kit TBirds get anything from 30 mpg to 60 mpg depending how you ride it . I er more towards the 30 mpg end hehehe . You will average about 40 - 45 mixing hard throttle and cruising I guess but it all depends on your riding style.
I had mine up to 60 -65 UK Mpg on a motorway run . Didnt last long though I got bored.