From Wikipedia: A turbocharger may also be used to increase fuel efficiency without any attempt to increase power. It does this by recovering waste energy in t

From Wikipedia: A turbocharger may also be used to increase fuel efficiency without any attempt to increase power. It does this by recovering waste energy in the exhaust and feeding it back into the engine intake. By using this otherwise wasted energy to increase the mass of air, it becomes easier to ensure that all fuel is burned before being vented at the start of the exhaust stage. The increased temperature from the higher pressure gives a higher Carnot efficiency. http://en.wikipedia.org/wiki/Turbocharger http://en.wikipedia.org/wiki/Turbocharger You're probably right that most cars aren't tuned to do that, though.