How does driving with the driver side window open affect fuel economy vs if all the windows are closed while driving on the highway? Would it be worse if all the windows were open?
A lot of this will depend on the design of the car, but I suspect the aerodynamics will be adversely affected (and the fuel consumption will increase) with the windows open.
Air resistance isn’t a simple matter of the air rubbing against the surface of the object, it is more about the air being stirred - air has mass and moving it about costs energy - the greater turbulence a body creates as it moves through a fluid, the more energy will be required to do so.
The Master sayeth thusly: