What effect is the spoiler having on MPG?

The Wikipedia says it's a Kammback. Maybe that's what you're looking for.

 

If I understand aerodynamicist Wunibald Kamm correctly (see NuShrike's Kammback link), a small change in the direction of the airflow could effect a large change in the following bubble of air. Changing the bubble could change the lift acting on the car, and thus the handling and sensitivity to crosswinds. It's not like the spoiler is doing any 'pushing down' on the car and is in imminent danger of falling off if you go too fast. The Prius no doubt spent considerable time in the wind tunnel, to minimise drag, but perhaps lift and wind vectors were not addressed as thoroughly.

 

You all might want to take a read on this page which discusses the Gurney Flap (aka Wickerbill for the NASCAS types). This is a small "flap" positioned at the rear of the spoiler. The result is to create a small separation bubble to form behnd the "flap" which forces the air flowing over the top of the wing to raise slightly thereby increasing downforce and planting the rearend more firmly.

If you look at the Prius cross-section, it is a wing and, IMO, even thought the BT spoiler is very small it would effectively cause the airflow at the rear of the car to push up and increase downforce. Call it what you like, mini-spoiler, spoiler, etc., it still looks like a Gurney Flaf or Wickerbill to me. B)

 

Yes, confusion certainly crept in, and I was trying to *straighten
it out* with my first post. Everyone must be too inured to the
BT spoiler ads plastered all over the header of EVERY page we read.
.
However, it installs too far forward to act as a Gurney flap.
It would be nice if the BT folks would chime in here and actually
describe the design basis, please.
.
_H*

 

I disagree.

Here's a Stanford Paper that discusses miniature trailing edge effectors (Gurney Flaps). Obviously, the Gurney Flap is designed to provide more lift at the rear to increase the downforce on the front of the car (its placed on the lower plane of the wing). But by placing one on the upper plane it would increase the downforce on the rear wheels. If you look at Figure 24 you will see that there is a fairly large area behind the Flap that is out of the airflow. I suspect that the dead space behind the BT mini-spoiler covers part of the original spoiler which doesn't interfere with the additional downforce.

 

It may look lazy, but it's going over 150 knots!