the Power being referred to is irrelevant of the means of propulsion. Basically the amount of energy needed to be put into the system to maintain speed increases as the cube of the speed. This is irrelevant to what the object is or what is powering it.

its all about relative air speed, the lower the relative air speed the better off you are.

but if your driving 55 into a head wind of 20 your relative air speed is 75
conversely if you have a tail wind of 20 your relative air speed is 35

since we have that issue of drag increasing as the square and power required as the cube you can see how you will never be able to gain anywhere near the amount of energy into the system from a tail wind as you will lose from a head wind.

I am not sure that you were actually taking the post in the wrong way, but it seemed as though you are implying that the statement I previously made may not be applicable to a motor vehicle because somehow the fluid dynamics of the vehicle aren't linked to the amount of energy required to move the car, however I am probably just misreading your post. and my reply then will have been all for nothing.