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Originally Posted by MINIz guy
I don't know enough about flow to really know how it affects drag. I'd expect the lower amount surface area the air stops on to have a big effect, but you could be correct.
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I don't really know squat about it either but it's pretty clear that the low frontal area of the OEM longitudinal rails will have little drag. The drag of the spaces underneath the rails is probably greater (pressure differences create drag - layman's description!). But even that is pretty low.
Removing the rear spoiler probably makes more difference at speed but it would make the car look neutered...
Adding lateral rails can typically add 5-15% to most car's drag coefficients.
Roof Racks a Drag on Fuel Economy | Berkeley Lab