I hve a GEBE 35cc 4-stroke, it's rated 1.1 continuous/1.6hp peak, and it cruises at 27mph (which would seem to be the "continuous" estimate). Plus, (with the same single gearing) it can pull HARD up most hills. My guess is it's not using very much of that 1.1 hp over level ground, even at 27 mph, which is the limit imposed by the engine's RPM setting.
From what I've read of bicycling (without engines!) a rider putting out ~300 watts (per hour) can be going 25+ mph, for a 25 mile time trial.,,, -but not a lot of people around can do that. ....Most typical people cannot pedal any "regular" bicycle faster than 20 mph for more than a few miles, and that takes something around 200 watts/hr.