I tested the open-ended kammback.
I saw a 1 MPG increase from 19.6mpg to 20.6mpg and that's good for about 5% better.
I wrote big thread on it hear:
http://ecomodder.com/forum/showthrea...tml#post561305
5%/1mpg was actually more then I thought it would be so I'm pretty sure I'm going to do this - at the very least make a less permanent one that can be attached before my road trips and then removed when done. We'll see.