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Old 12-08-2010, 03:16 AM   #13 (permalink)
Frank Lee
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Join Date: Jan 2008
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Blue - '93 Ford Tempo
Last 3: 27.29 mpg (US)

F150 - '94 Ford F150 XLT 4x4
90 day: 18.5 mpg (US)

Sport Coupe - '92 Ford Tempo GL
Last 3: 69.62 mpg (US)

ShWing! - '82 honda gold wing Interstate
90 day: 33.65 mpg (US)

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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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