I've confirmed 55 on a flat road on my Fox with GPS. I've confirmed 48 with a car trailing me. Damn thing was terrifying to ride at that speed. 12/15/15/54 gearing. I kept the jackshaft so I can play with gearing and possibly go to a 2 speed clutch in the future. Anyway, on the current gearing with the 15.4" tall tires, that's 5400 RPM to hit 55. To hit 75 I'd have to pull 7400 RPM, about where the valves would float. Arguably possible, not terribly likely with my largeness on it unless I'm going downhill. I'd worry about the engine block at that RPM as well.
Bike has an ungoverned and modded Briggs model 13.