Working out the frontal area isn't that difficult. Very roughly it's overall height * overall width, minus ride height * overall width. Plus 2 lots of ride height * tyre width.
As well as the Z4 I used to have an E60 520d, which I managed to get to 50mpg on a run. I wondered how this was possible until I discovered that the drag coefficient was only 0.25 and everything forward of the bulkhead was aluminium. Not bad for a big car.
The Z4MC drag coefficient of around 0.35 explains why the top speed isn't that great. I haven't VMaxed mine, but I reckon it would struggle to get to 170, 165 is probably closer.
