Does The Spoiler On The Gen 3 Prius Help Or Hurt Fuel Economy?
 

Is it a cosmetic piece that destroys the smooth departure of air from the tail, especially if one adds a boat tail? Few of us will ever drive fast enough to need the down force. Has anyone removed the spoiler, and replaced it with a smoother piece? :confused:

Most people think of spoilers as Nascar and down force and the huge wings on riced out Honda civics.
A spoiler in its true form breaks or changes air flow over a surface. The spoiler it's self as an individual element increases drag but When applied properly they can reduce overall drag on a body.
Since the prius gets something like 50 mpg highway I would say it's likely being used to reduce overall drag.

What oilpan4 said above. And also, the tail end of the current Prius has some pronounced "box cavity" elements: a void between the tail lights and decklid and integrating the bumper cover sometimes to its base and rear diffuser. Google aerodynamic rear box cavity experiements. There are formal papers out there.

I could take the spoiler out of the picture by covering it and the wiper with a "profile" compliant plexiglas cover blending into a "profile" compliant Boat Tail:

Yes. But you needn't. Building over the backlight on a Prius, which is so well optimized already, compromises functionality with little hope of much benefit. Both visibility and hatch access will be adversely affected. Better to start your tail at the spoiler. And you might amend this bottom part. The angle of the diffuser seems to quick. You will want a more gentle rise, if other projects are prescriptive.

Yes, optimized for the shorty tail on the Prius,

But, the spoiler will detach the airflow and send it high.

I need a ramp angle to keep from dragging on driveways.

Well, if you are determined, that's great. Experiment away! Please post pics and results and stuff. It will be fun and interesting to see your results.

Thank you for your suggestions.

It's not a spoiler, it never goes above horizontal. It's a reflex 'tearing edge'. Newer models have it on the bumper ends as well. You're trying to fit Thee Template. Just put a wedge piece in the hatch opening so it doesn't close as far, and remove the 'spoiler'. Said wedge could be the front lip of a boat tail with some added side pieces. The top of the opening may or may not need a shim.

For the departure angle, skegs hinged at the bottom front with skateboard wheels embedded in the tips. If not an entire inline skate.