Rear deflector...posible mpg gains?
 

Would this work?

I have heard other people say that the main reason for those deflectors is to help keep the rear window clean. I've seen them come stock on the back of a lot of GMC Jimmys etc.

I am interested in what everyone else thinks since there is a similar "ductlike" deflector built in to the rear roof section of my 87 civic hatchback. I have it plugged up right now.

I suppose one could imagine that they might help to fill the low pressure area behind the vehicle, but I don't know what to think. Depending on what you all say I may unplug mine again.

it could fill the low pressure area, or it could be a little sail.... had to say without a wind tunnel

On planes they use things like that when a curve is too sharp and to redirect the laminar flow so it stays connected. But that looks more than a bit extreme. Something like that above the rear window of a civic sedan would probably work to keep the flow connected. I think mitsu used one on the evo too. I think on planes they use them between 12 and 25 degrees of variance from the flow. I was going to try one on my short-lived civic 2 door.

I'm going from memory and I haven't read anything about it for 3 months. I bet our resident HPV builder knows the real answer.

Where are ya trebuchet, chime in here.:)

I agree with Erik, the main purpose of those deflectors is to deflect air downwards to help keep the rear window clean. In the process of doing so, they create lift and probably more drag than a small downforce-generating spoiler. Whether or not they'd actually help simulate laminar flow is anyone's guess. IMO, VGs would be a more passive approach to attaining the same goal.

that thing looks a bit to dramatic to give me the idea that it would help, but a test is the only way to be sure.... perhaps something similar to that would help it it was placed behind the hatch rather than scooping around it, so that it wouldn't add any frontal area... but than again most hatchback i've seen that seem to be geared towards economy use as straigt lip that continues the lines of the roof... sometimes it angles slightly upwards... although that might be for downforce and stability.

filling in the wake is not always a good thing... you want the wake to be shaped like a small cone, but you don't want the air to go over a steep edge so that it creates a swirl that sucks at the back of the car... that's why most cars have a sharp trailing edge and some cars wich seem to slope down all the way, like a VW beetle or some older porches have not so good drag quoefficients and benefit from big spoilers that sit in place of the average sedans trunk.

They do keep the rear windows and tailgates clean. Improve mileage? I don't know.