The shape of the 911 is such that because of its sloping rear, a spoiler is needed to divert air in order to avoid  rear lift (hence the motorized Carrera spoiler) but at higher speeds downforce is needed (hence the GT, Turbo wings). It seems to be technically and ergonomically difficult to make these big wings to disappear completely when the cars are moving slowly.

The 991 is a 911 shape with a higher rear that should aid aerodynamic behavior but I don't think it could made any higher, because that will change the whole 911 concept. Even now Porsche friends complain that the 991 rear is too high.


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"Form follows function"