DPV: a clear illustration of
roof spoiler differences... (NOTE that trunk spoilers can be angled upwards and reduce drag... just talking about roof spoilers here...)
If you look at, for example, Volvo's own C30 "Efficiency" model, among several aero enhancements, they
reduced the rear spoiler angle from the regular model, essentially turning it into a roofline extension (extending the slightly downward slope):
(
source)
vs. original model with higher cD:
