I have a 2009 Yamaha Royal Star Venture with Hannigan conversion kit. Without a passenger running about 65mph I get 28mpg. At 70, around 26 mpg. Higher than that it can vary anywhere from 20 to 25. I know 80 is about 20 to 21.
Pulling a trailer with about 300 lbs and a passenger 140 lbs I was getting about 23/24 mpg running 65 to 70 and 22mpg 70 to 75.