This may seem like a stupid question, but I have been wondering about it for a while.
In a car, to get to better fuel effeciency, the transmission changes gears based on the RPM's of the engine. More throttle means more upshifting, less throttle means down shifting. Why not employ this in an airplane engine?
A318, A320, A332, A333, B1900, B722, B732, B733, B734, B735, B737, B738, B772, CR1, CR2, CR7, CR9, MD80, MD81, MD82, MD8