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It's all about wake reduction, and to max that, you want no wake on the lid, leaving only the rearmost vertical area- tailgate, bumper, etc.- with wake behind them. If the lid has too much angle, flow separates on it and the lid becomes part of the wake zone. If the lid has not enough angle the upper part of it won't be part of the wake zone (good) but the rear area won't be minimized hence it's overall effectiveness won't be maximized.
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