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Originally Posted by Smoky
This is from another post I did and have already been challenged on it but here it goes:
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Prepare to be re-challenged!
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That fast back crap won't help a bit, it will only add weight. A pickup truck already gets that effect by the turbulant air behind the cab with no extra weight.
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That's an extreme statement, unqualified, & a massive generalization to boot. At speeds where aero matters: tapered bed cover > tonneau cover > open bed > full width&height cap.
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A truck cap with the extension off the back would also do.
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If you mean one of these, that's not true, unless your extension is tapered in plan & taper, like the fastback "crap".
In fact it'll likely get
worse fuel economy than an open, uncovered bed because the flow separates from a point that is nearly the maximum cross-sectional area of the vehicle, with nothing downstream to reattach to. So the trailing wake is almost as big as it can possibly be.
I shouldn't have to say this, but note: I'm talking highway speeds, where aero resistance dominates as the largest proportion of energy requirements.