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Well, those stems cause interference drag between the body and the stems and between the stems and mirrors. Just like interference drag is created between struts and wings of a biplane.
Also lets not forget drag equals Cd times frontal area. So in order for the mirrors to be "aero neutral" they'd need to reduce the Cd by as much percentage as they represent frontal area of the total car. Looking at the picture I previously posted, That would be far fetched to say those mirrors are completely enclosed in vortices and do not stick out at all in airflow that would otherwise be mostly undisturbed.
An SAE paper on the Volt aerodynamics would probably shut me up on this subject, and if the mirrors are really aero neutral I'm pretty sure there would be such a paper published. But until then I'll keep on doubting.
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