Civic Wagon rear window deflector

[Gasoline Fumes]

Good or bad?

[Wonderboy]

Duct tape!

[wyatt]

It is put on to keep the rear window clear of dust. It probably adds (a small amount of) drag to the vehicle, but allows you to see out the rear window... If you can get your hands on a Hucho book, you can see where they have done tests with vehicles on dusty roads both with and without similar devices, and the ones with them have a clean rear window, the others are coated in dust.

[Dr. Jerryrigger]

@wyatt that's interesting.

That is not going to make any noticeable difference ether way, and the dust thing is cool, so if it was my car, I'd leave it be.

[lunarhighway]

the fact that air is caught and redirected by that thing will add a little drag, but as that air is expelled where you'd normally have a turbulent wake it might actually reduce the size of the wake slightly,

also it doesn't really add any frontal area.

on the other hand if the plastic bit is removed the downsloping roofline looks like it will be aerodynamically favorable.

if you can take it off fairly simple that that's worth giving a try, if not, enjoy the clear rear view

[wyatt]

LunarHighway makes a good point about it not adding to the frontal area of the car. I would say, 1. Look it over good, see if it is EASILY removable and EASILY re-applied. If it is, you can take it off and see if you notice a difference in the cleanliness of your rear window (this is important, since you will not be able to notice a difference in your FE, even though it will be ever-so-slightly higher). In a city, you may not have dust related problems, at least not to the degree you would on gravel. You may get by with washing your windows every time you fill up and be just fine, in which case, I would leave it off. (I would only remove it if it's easy to put back on.)
Where I grew up, I would have loved to have something like that! We lived on gravel roads (5 miles in both directions), and it was always hard to see out the back of my little car.
My mom's Saturn Vue has a deflector that does add to the frontal area, and keeps her rear window nice and clean. It's actually one of the reasons she likes it so much (the car not the deflector), because the back window never gets a lot of dust on it.
Older Jimmy's and Blazers had them too, just a flat sheet that stuck up into the wind a bit to re-direct air down over the rear window. At least my mom's Vue and your Civic have a radius on the redirection, meaning less drag.