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Old 01-09-2008, 07:23 PM   #1 (permalink)
MetroMPG
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Location: 1000 Islands, Ontario, Canada
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Blackfly - '98 Geo Metro
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How to decide whether to remove rear spoiler to decrease drag, improve mileage

This question has come up a number of times: "Does a rear spoiler lower my mileage?"

I'm wondering if we can come up with a few guidelines about whether or not to advise rear spoiler / wing deletes, considering that ....
  • some OEM spoilers reduce both drag & lift:
  • others are designed to reduce lift at the expense of increased drag;
  • and still others increase drag solely for the happiness of their owners and the joy they bring their viewing audience:





Also consider that while some OEM & aftermarket wings & spoilers improve high speed stability, the type of person who is going to remove a spoiler to increase efficiency probably doesn't drive at speeds where reducing rear lift is an issue.

Generally speaking, I'd say that spoilers which extend the decklid, or sit flush on top of it at a shallow angle should be left on. These are the style typically added to high efficiency vehicles to promote clean flow separation, or to enable flow reattachment at the end of a 3 box shape:




The Renault Logan Eco2 sported a deck lip extension and other mods to increase FE.






Honda Civic Hybrid has a rear spoiler not found on the non-hybrid version.



Is it reasonable to say that spoilers (wings) that stand off the rear deck should be deleted for better mileage?

EG:




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