It does help, I'd say about +10%!
Mine cost me about $100 but it took some doing...
My first issues were weight and bed access, only a made-for-p'up truck cover would help relieve these issues. Even if costlier it's ready made, or so I thought.
Then the biggest two things are one water pooling or puddling when it rains...
It's not so bad until you got a 5 gallon puddle in the center of that thing
And aerodynamic down force behind the cab.
You see, as you drive there is a STRONG down force of wind in the rear.
I was amazed, so be prepared to play but a few hours and some materials gtg (I didn't have to pay for the plexiglass thou), the only other option is a decent cover cost like 4-5-600....
I heard you don't want to spend that much (neither did I) but know that a ready-made $200 cover will not do it, not by itself. At least that's what I learned.
You could, however, try some this ready-to-go cheapness:
http://www.jcwhitney.com/Soft-Tonnea...Ns=p_MIN_PRICE
(First one 65 bucks or so)
Not sure how it will act once you start driving, it also might require reinforcement...
As for it getting rained in, first wait and see...
Then maybe drill a tiny hole in center, pin hole?
But I can not say if this will compromise the material now...
This would have been my cover of choice except I have a toolbox.
So I ordered my Tarpeez from here (but keep reading):
http://www.supertruckusa.com/product...z&submit=Go%21
And this is the OEM's site:
http://www.tarpeez.com/
Unfortunately the OEM pictures do an injustice, once you go driving the wind pulls that cover right down to almost bed level so I had to reinforce mine from below (it does fine now).
The pics and a bit better description are towards the bottom of this:
http://ecomodder.com/forum/showthread.php?t=1109
Other notes:
The DOWN force is strongest near the tailgate, almost non-existent near the cab.
Well, at least on mine it was (club cab).