I asked aerohead about this and this was his reply:
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(2) Sepp has reported that an open shell did not offer as much drag reduction as a closed shell on his Nissan truck.Bob Parson's open boattail for his Vanagon did not return as high a savings as the closed unit on my Transporter but his "lifestyle" dictated an open plan and he was happy to live with the difference.Open units may allow"circulation" which always has to be paid for at the pump.
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So I closed it. I think it looks better too.