[aerohead]

With the original modifications, the upper contour of the boat tail best matched the W.A.Mair, and FKFS (Kamm) 1/2-body silhouettes.
Maximum down-slope angle is 19-degrees. Diffuser upsweep is 20.5-degrees.
The 'highest' streamline filament traveling under the Impreza begins about 16.1-inches above ground level.
Along with the other mods, the tail gets the car to approximately Cd 0.214.
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For the second-generation, #4 configuration modifications, the upper contour of the boat tail is almost an exact match to W.A.Mair's and Buchheim et al.'s 1981 VW Flow-body tails.
Maximum down-slope angle reaches 23-degrees. The diffuser angle is 5-degrees upsweep.
All mods contribute to an approximate Cd 0.1764, and achieves an almost 21% improvement in mpg.
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Averaging for all configurations, the delta-drag/delta-mpg relationship works out at:
delta-10% drag = delta-5.15% mpg ( assuming constant BSFC )