The stock CB 650 inline four was a lean burn EPA engine. 50MPG was "normal". However, it was likely measured on the highway in top gear cruise. Pretty clear that using lower gearing will make MPG worse, as the engine turn more per mile. But, your driving habits will be the dominant factor in what you experience.
And then there are the owner "improvements" which generally disregard any before-and-after measurements to determine actual performance change.
There are a lot of posters that tout their improvements with the yardstick of "no problems", as the ultimate benchmark. At least no problems they chose to perceive in their driveway cruise.
Cheers,