[aerohead]

The graph only illustrates 'Road Load Horsepower',where the available power delivered to the traction wheel(s) is absorbed.
It has nothing to do with thermal efficiency of the prime mover,accessory losses,or power train mechanical efficiency.
What is does show,is that,at highway velocity, aerodynamic drag constitutes the greatest portion of an automobiles power requirement,and is the greatest source for energy efficiency improvement.