Much of the increased fuel efficiency came from the addition of electronic modules to control nearly every system in the automobile… engine control, fuel delivery, transmission control, power steering control, etc, etc, etc. Each of these advancements have added a few percent to the overall efficiency.

For example, power steering used to be accomplished by a hydraulic power steering pump on the serpentine belt on the engine. When you were driving on the freeway at 75mph, you were barely turning the steering wheel. Most of the power to drive the pump was unused and wasted. Electronic Power Steering (EPS) instead senses the need and then controls motors as required, providing roughly 3% overall fuel savings.

This same approach has been applied to every system within vehicles. Rather than add weight, electronics increase the fuel efficiency while reducing weight. For example, the EPS modules and motors weigh less than the hydraulic power steering pump, hoses and hydraulics that used to be driven by the serpentine belt.