I average right at about 50, but I get into the 60's on the highway.
Motorcycles are about as aerodynamic as a sideways brick. Speed kills the fuel mileage. Doing 80 on the highway is going to use tremendously more fuel than doing 65, because each extra ounce of speed is more force keeping that sideways brick barreling through the wind!
I find I get my best fuel mileage on state highways with limited stops, riding easy, with few hills. In other words, similar to riding 55 on the interstate. I've gotten into the mid 60's that way!
According to fuelly, which I have been using for a few weeks now, over the last 1,100 miles I have averaged 48.4 mpg, alot of that being two up, that's with a tall windshield, and I ain't exactly 'light', so I cannot complain!
Fuelly also keeps track of the price-per-gallon each time I fill up, and over the last thousand miles I've averaged about $0.07 per mile in gasoline, not too shabby looking that direction either!
http://www.fuelly.com/driver/romansfiveeight/vulcan-900-classic-lt
-John