Originally Posted by morfinx
Why would that affect fuel efficiency?
I am not sure if the sport button in the Z4 does the same as in the 1-series M coupe. If it does it would let the turbos run at higher "idle" speed and thereby reduce turbo lag. This creates more loss and worse mpg in some conditions.
And when you spin the wheels in sport+ you also get worse mpg. This would not happen in sport mode.
In DCT cars the gear shifting points and clutch speeds are also different in different modes. Due to the imperfectness of our gasoline engines, we get better mpg by selecting the highest gear possible at any given cruising speed.