*if the old metric held true,then we'd look for a 5% mpg benefit for each 10% drag reduction at 55-mph according to GM Laboratories.But that's assuming a constant BSFC which is hard to know.Gino Sovran warned that we could lose up to 40% of the streamlining benefit without proper gear-matching.
*if there was separation along the sides,the low pressure of the separation line would be transferred to the wake,lowering the base pressure and increasing the pressure drag.
*I've got some of that happening on the T-100 and the only way I can fix it is to make a deployable diffuser,like GM's Epcot car, which can drop down to 2.5-degrees up-slope,only on the open road.(cable/pulley/hand-brake mechanism).You can see how 'nervous' the flow is under there in the video.
This would be a 'safe' contour for the sides.A lot of air is shunted over the roof on the CIVIC,leaving a more feeble flow on the sides which cannot tolerate much of a pressure increase (plan-taper) without triggering separation.
Here,Fachsenfeld has used a very conservative contour for the sides of his K-form car
All the 'Kamm' research was done with very gentle plan-taper
The Cd 0.12 MG EX 181 of 1957 used this gentle teardrop taper as well
