You can actually find a mirror wind tunnel study on the internet which I remember to be volvos. Basicly further the mirrors are from the a pillar corner the better up to certain point where mirror leg frontal area eats the fuel consumption benefits, but also makes car too "wide"
http://publications.lib.chalmers.se/...ext/143193.pdf
Also you should notice that mirror is hanging lower than the hood so they cleverly direct most of hood airflow over the mirror
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