BMW has been pursuing their efficient dynamics program for a while now. One example is where the application of stop/start technology, a feature where the engine is stopped and started by a motor/generator instead of the standard alternator and starter. Along with this, a larger battery is capable of storing more power and accepts the electricity generated from regenerative braking. Already added to the 1 Series, 3 Series and 5 Series, stop/start has now been moved to their Z4 sports car.
Different from the others, though, the Z4 also has a 'friction optimised' rear axle which incrementally improves the fuel efficiency of the car. All of the changes mentioned above conspire to add a few percent of increased fuel mileage to the car. Every little bit helps!