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Welcome to the site. As for the air deflector, you might see what happens without it. As for me, I'd leave it on. I had a '79 Malibu station wagon years ago, and after I added the spoiler to keep the back window clear, I noted an increase of just over 1 mpg as well. I'm not the aero-expert of the group, but I just figured (at the time) that it forced a little bit of air into an area that was sucking huge amounts of air as a sort of wake.
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