My MPG are the same as yours with a large Memphis Shades windshield and stock everything else. Well with the exception of Mustang seat and aftermarket grips. When I ride to work and back at 100 miles round trip and 90% interstate at 70-80 MPH I still average around 50 MPG.I'm getting about 55 mpg when I cruise my 750 Spirit on the back roads about 50-60. But when I spend about 20% of my time on the highway going 70-80mph, it drops to about 50 mpg.
I also have Cobra pipes with the carbs re-jetted. Sounds awesome.