Most of the membership of this forum is from the UK, I am guessing, and the ethos of car ownership seems to be a little different here to in the States (particularly Cal). No one buys a sports car for economy or emissions here; they used to buy diesel, now they buy hybrid/electric, which is why your question hasn’t had a serious reception.
If you really want to maximise economy/emissions from your Z4 drive it more carefully, make sure the fuelly bits are in tip top condition (VANos, DISA, ICV, sparks, filter), don’t run the aircon, use quality fuel, consider a more fuel efficient map, change the tyres as you say.