I think that over a certain speed, fuel economy goes way down.
This is because:
For high velocities drag will vary as the square of velocity. Thus, the resultant power needed to overcome this drag will vary as the cube of velocity. The standard equation for drag is one half the coefficient of drag multiplied by the fluid mass density, the cross sectional area of the specified item, and the square of the velocity.
https does not work.
I installed a wood burning insert into our fireplace. It has a large glass door so you can still see the flames for ambiance. It is 70% efficient, using a blower to circulate heat and is designed to reignite smoke to reduce particulates. I gather wood locally from trees my neighbors cut down after summer storms.
So I can heat the house in the event of a power outage. I also have an inverter so I can use my prius as a generator to run the blower, router, wifi.