Gas mileage has more to do with your right grip than anything else, and it's much worse at highway speeds with any luggage sticking out there. The further it sticks out, the worse it will be. Some of the bags out there make the bike 4 feet wide.
What he said.
DL650K5, Race Tech Forks, Scotts Stabilizer, SuperBrace, Staintune Exhaust, GSXR Calipers, Fastway EVO III Pegs, Givi Crash Bars, Motoagora Bellypan, TouraTech Rad Guard, Givi E21 / E41 Cases, Givi E52 Topcase, Cortech SuperMini Tankbag, Suzuki Euro Screen / Givi Touring Screen, Madstad Bracket, Kaoko Throttle Lock, Suzuki Heated Grips & Handguards, Dual PowerLet Outlets, Pat Walsh Dash Shelf, V1 Radar Locator, Garmin Zumo 550.