Nothing noticable for mileage, but I'm never on any straight runs that give consistent mileage anyway. But I would guess w/ the arodynamics of it, I would think that at speed... mileage would increase before it decreased.
Yes, it is a drastic improvement over the much smaller spitfire, and a friend says it is a somewhat of an improvement over the OEM Suzie windshield that he traded out for it. But don't let me mislead,.....when its 40'ish out there, this ole guy is riding in something w/ a heater in it !