Welcome Budman, be prepared the railroad may be calling you back from time to time, to haul broken trains back it with your Rocket. OK I know it's got gobs of torque and horsepower but it ain't gonna pull no train. My wife has a 1300C on a trike kit and she absolutely loves it, the VTX is a nice bike. Keep the rubber side down my friend.
Heretic, the wife gets close to 33-34 with her 1300 VTX trike and it weighs in at close to 1100 lbs. So, I would think 40 is about right for average fuel mileage on a regular X.