The amount of gas in the tank will greatly effect your tank milage. Duh, right? Well, if you do not fill the tank to the same point every time, you'll get more or less from your "tank" of gas. Some people insert the nozzle all in and let the pump kick it off as full. I pull the nozzle out a bit and visually fill it to the same point each time.
As for running a vac. petcock bike bone dry, believe me, if you don't PRIME the carb before trying to restart, you will most likely kill the battery before it even tries to start.