I had two full Wings before my F6B, an '08 and a '12. I got a fuel milelage range of 38-42 pretty consistently on all three. My F6B tends to stay on the lower end of the range because I tend to ride it harder just because it is so much fun!
[SIGPIC][/SIGPIC]
I generally break 40 riding two up with the wife. I confess to not riding real agressive when she is on. The worst I've gotten is 35 on a 160 mile trip at 4 in the morning in 1 hr and 50 minutes. Generally riding by myself yields about 38 mpg. As for the windshield thing , you are correct. Bigger the shield less the mileage.
I go 385 lbs so I could be called two up alone which is the only way I ride now days.I am 6'5" with a 19" Baggershield up front.I have never gotten below 40mpg and best was 45 with a tail wind.
A lot depends on how hard you ride.