This had been gestating in my mind and I went ahead and did a workup on it:
In Hucho et al.,SAE Paper 760185, Figure 24, and Hucho et al., 2nd-Edition, Figure 4.62, page 155, they lay out the effect of various aft-body configurations on total drag, for the Golf/ Rabbit, and Jetta.
There's enough visual information in the two figures, with which to reconstruct the actual 'shapes' of the two cars, to complement their associated drag coefficients.
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The Jetta is just the Golf, with a little more than 10-inch ( 254mm ) trunk extension added to the rear.
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An eleven data-point scatter-plot is provided for the Jetta, illustrating the Cd at each configuration, ranging from Cd 0.407, to Cd 0.365.
I drew out six (6) of the configurations.
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* If the 'step' is removed from the Jetta, it produces a 'fastback' with a constant rear slope angle of 33-degrees, placing it in the forbidden region of bistable flow, which on the sibling Golf, generated up to Cd 0.44.
* If the step of the 'Basic Shape' is re-installed, the drag falls to Cd 0.407.
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* The addition of the Basic Shape notchback step is clearly providing the same function as what one would seek from the addition of a rear spoiler.
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* If 150mm of additional height is added to the Basis Shape, the drag falls to Cd 0.37.
* If 450mm is added to the Basic Shape, the drag minimum of Cd 0.365 is achieved, according to the scatter-plot.
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* Adding 550mm to the Basic Shape degrades the flow to Cd 0.38, at Shape B.
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The drag of the Jetta can be altered by delta-Cd 0.075 simply as a function of the rear body slope angle / step height.
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In Hucho's Figure 4.60, by Buchheim et al., the 'curved-roof' wind tunnel model also registers a delta- Cd 0.075, also with the single rear slope angle parameter, by sweeping through 200mm of variable step height, indicating angles from 36-degrees, to 13-degrees.
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If you're out and about, and happen to spy a late-model Accent, Rio, or Mirage notchback sedan, imagine what the rear slope would be without those boot/ trunk lids.
