I have just tested my Honda Insight in three different rear-end configurations:
- no spoiler and fins
- with spoiler but no fins
- with spoiler and fins
The car:
The results:
The groups of bars show how many reading locations were taken - so three locations across the spoiler (LHS, middle, RHS), two in most other regions (LHS and RHS) and just one at the centre-forward part of the roof.
As can be seen, the presence of the spoiler alone dramatically increases pressures immediately in front of it, with its influence occurring as far forward as the front of the roof!
In every location on top of the car, the presence of the fins improves the action of the spoiler.
Highest pressure in the wake occurs with no spoiler or fins, but the presence of the fins increases base pressure a little over having only the spoiler.
(Note that my overall drag measurements show the fins + spoiler reduce drag, so the increased pressures on the rear window are more than making up for the reduced pressure in the wake. With the spoiler fitted, the wake is the same size as the standard car - ie the trailing edge of the spoiler is no higher than the standard car.)