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Only way to know for sure is to test it.
I've found air dams have helped on multiple vehicles, even ones that extend well below the lowest underbody component: Firefly/Metro, Mirage, Micra, Miata. (Hmmm... lot's of "M" cars!)
I tested the air dams on these either with multiple coast-down runs, or with multiple constant-speed MPG runs. Always tested on the same route, same day (same ambient conditions).
But on one vehicle I tested, an air dam appeared to make things worse: 2004 Toyota Prius.
Moral of the story: make one and test it out!
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