I can only guess that what is to follow is a direct result of not knowing much about turbos:
My understanding was that because a turbocharged engine will produce an equal amount of power more efficiently than a similiar N/A engine that there could potentially be a higher efficiency at highway speeds. For example, if it takes 100HP to keep my truck moving at 65MPH on the freeway (~2100rpm), the turbo'd engine will be able to produce that 100HP more efficiently using less fuel.
