Ah ha! Thank you freebeard, I didn't know it was a specific spoiler.
It seems to me that if the length was optimized for streamlining, it would decrease Cd.
Very close.
Maybe the spoiler needs more angle in order to create a vortex so the air "rides" over the trunk?
Sorry if this is off-topic
Edit: Cd, thanks for the info and link posted above
That is what I was attempting to show.