Gas mileage can change without changing much. My Rocket with all the stuff on it, riding solo on the highway gets around 36 mpg. Take off the tour pack, same ride will be just under 39 mpg at 70 mph. That said, a trip with my wife with touring pak, pulling a cycle trailer, 50 to 60 mph on 2 lane state roads will drop my gas mileage to 28 mpg. All these changes and riding very conservative, I really can't notice the engine working hard. Pulling a stupid trailer, I expected the bike to be a dog. Not at all. Just changed the usage.
Getting just under 40 mpg riding with a group of Harley's requires very little throttle. Riding with another Rocket on the interstate is another story. Much faster, and more gas, not over 34 mpg.