[j-c-c]

IMO, a rear spoiler is supposed to "spoil" air flow, and therefore increasing the local air pressure upon a surface up stream, to intentionally increase DF. Increased drag is thought to be an acceptable trade off in most cases.

A flat added extension (trunk?) should, again IMO, maybe better be called a "smoother (or divider?)", which might reduce drag. If a "smoother", just by forming a hard boundary between a higher pressure area and a lower pressure area in space, will also increase potential DF, with little drag increase, if any.

Doesn't seem to me that a typical rear spoiler often reduces drag, leading to better mpg.