I've been averaging 75 mpg on my 2011 G5.
All of this includes the initial 300 mile 40 mph max break in thru my present 1300+ miles and 65 mph speeds. Most of my riding has been in the 45-55 mph range.
On the headlight nacelle to the lower right of the speedometer is a round faced multi light gage.
It includes the check engine light that glows for about 4 seconds while the computer boots up and a Low Fuel light.
The low fuel light on my bike flickers during braking after about 150 miles but at this distance it is normally off. This 150 mile distance requires about 2 gallons of fuel to fill the tank.
Somewhere I read that the light really comes on after 3 gallons of fuel has been consumed so that would be at about 225 miles.
The tank holds about 3.8 gallons but RE recommends filling it before that is reached because the gasoline is the coolant for the fuel injectors fuel pump.
If you have a 400 mile butt you will just have to convince it that it needs to take a break and have a walk around every 150-200 miles.
There is no way my butt could sit on a motorcycle without a break every 100-150 miles and besides, IMO one cannot compare riding a motorcycle with driving a car.
Not only can you not move around much on a bike but for me, the mental concentration required while traveling at freeway speeds requires a rest too. Besides, when riding, getting to some distant place is not the goal. The pleasures and excitement of the ride is the goal and this includes stopping at places I would zoom past in a car just to have a look around.
Reaching the place I'm headed to is nice but it also is the end of the fun of getting there.
More than once I've found that the thought of the ride home would be more fun than sitting around with my relatives.