Druade is pretty much spot on, tho i wouldn't say that high of a spread. I'd say it generally seems to go from low 30's to low 50's with a few exceptions. But most seem to never go higher than low 50's and never lower than low 30's. Myself, i have the 1700 and usually see around 37-40 in the city depending on how hard i ride. I'm not a crazy rider, but i also often ride in a fairly spirited manner. Almost never very sedate like a lot of HD guys do. Thats city with a lot of stop and go. And stop and go is by far the #1 thing that drops the Tbird's MPG, and big time. On the highway it goes above 40 often up to mid 40's. Riding highways/freeways with not stopping to speak of will get great mileage. I recently went on a camping trip with a friend who rides his HD like a old lady. I had to hang back to keep from losing him, so on the freeway we rarely exceeded 65 and often down to 60 and even 55. That weekend i got 50 MPG, the highest i ever got.
So the bike really swings wildly on MPG depending on what type of riding and how you ride.