My mileage went up with the much larger Windstrom Manta. With a 16t front sprocket, I am getting 48 to 50 with a topbox, I fight the wind regularly, I ride it pretty easy going through the gears staying under 5000 in town, but I usually cruise at 90 and above, indicated, on the slab.
Oddly I think my mileage also went up after getting the Givi Windshield. Seems like the larger screen would reduce it but for some reason it seems to have improved by a couple MPG.
2007 DL650 with Givi windscreen, Madstad, Givi E33 topcase, GIPro, Symtecs under Progrips 714's, Sargent seat, Richland Rick fork brace, RacerATB corrected gauge face, Stebel Black Max, Aprilia mirrors, Suzuki Hand Guards, Battle Wings, Shermworks Open Road Stand.