Tonneau cover extension/spoiler
 

I washed my truck today and noticed that my tailgate was the dirtiest part of my vehicle. I think this is a bad sign for aerodynamics, and indicates that my airflow is not detaching cleanly at the rear. I have a tonneau cover and want to try extending it about 6" either straight back or with a small angle.

Any advice for the angle of such a spoiler?

This is a rough idea of what I have in mind.

http://i1152.photobucket.com/albums/...pse9df1aae.jpg

Not my truck but the same cover I have.

If you get that to work out for you please let us know.

I did not see any mpg improvements when my adjustable rear spoiler was flat out, but the transition was not idea, and the devil is in the details.

However, I believe the theory you have is sound, it just does not add to down-force as I once desired, in fact I think it contributed to uplift if anything at all.

See this thread:
Rear Spoiler on Pick Up Truck - Experiment - Page 2 - Pelican Parts Technical BBS
NOTE: The adjustable angle is adjustable no longer, the piano hing has long rusted into a set position. Photos are almost 5-years old now.

There was some discussion in the forum of including the tailgate and sides into the equation.

This was a mark-up I did for such a tailgate.
http://ecomodder.com/forum/showthrea...tml#post381583
More here, including actual built projects + Patent.
http://ecomodder.com/forum/showthrea...k-26367-2.html
http://ecomodder.com/forum/attachmen...p;d=1373575937

advice
 

The base pressure of your wake is determined by the pressure at the line of separation,top,sides,and bottom.
An upper spoiler will do nothing to lower this vacuum behind the tailgate as you've still got 'forward' separation aside,and below it.
Either a proper boat tail or box cavity is the only thing which is capable of any meaningful drag reduction.
Also,the end of the tailgate may already be in the 'ideal' position with respect to the 'roofline.' A comparison to the Aerodynamic Streamlining Template tool at the top of the Aero Forum would tell you where you are now.

Well that answers that question ;) Thanks for the knowledge

It does answer your question, but if you lower your expectation to just wanting a clean release of air off the rear of the bed cover (which is a step in the right direction), then following what the latest truck designs are doing may help.

PickupTruck.com - First Look: 2009 Ford F-150
http://www.pickuptrucks.com/trucks/I...ord/f15019.jpg
I think that you were just asking too much of too little in your original post.

Here is an aftermarket installation with a camera:
How To: Install Tailgate Spoiler With Back-Up Camera - DodgeTalk : Dodge Car Forums, Dodge Truck Forums and Ram Forums
http://i27.tinypic.com/2z74e8w.jpg

Lots of other types too: "tailgate spoiler"
https://www.google.com/search?q=tail...h=677#imgdii=_

latest truck designs
 

I just wanted to mention that the automakers have commented that the tailgate spoiler itself has very little if any actual streamlining benefit.What they're hoping for is that Joe Six-pack will remember to keep the gate up where it can do the most good compared to gate down,or gate off,which I see locally on a daily basis.
You can bring a brain to a cerebrum, but you can't make it think.

I imagine an extended spoiler would be effective if it was integrated into an aerocap. Similar to the rear spoiler on the Prius.

I noticed a pick-up truck parked near my house yesterday, I think it was a recently produced F-150.

The rear tailgate spoiler was complemented by a matching plastic cap on the top of bed each flank.

The overall view of a "U" shape comprising of horizontal 3-inch wide plastic cap gave a very solid feel and perhaps provides a flat surface for attachments, loads or just to sit on.

extended
 

Yes,anything which extends the length along the streamlined contour is beneficial.