It does help to open the sidescreens (if fitted) some 2-3 cms to reduce air suction behind the windscreen (as this suction draws the fumes into the cockpit).
Fumes should be almost not naticable without side screens fitted - my experiences with several cars yet.
And: yes, spare wheel-less cars seem to suffer more from that ugly smells (which cause bad headaches...) due to different flow behaviour / turbulences over the rear panel.
Hope that helps?!
2015 Roadster 3.7
