Yes it will help with fuel economy. I'm not sure how the back pressure helps to build torque, I just know from experience that it does and when there is more low end torque, mpg goes up. test it yourself... fill up and drive a 50 mile loop and measure your mpg, then mod the exhaust and go on the same loop. you'll see the mpg go up. and I wouldnt be supprised to hear that you scratched your tires on the way out... thats what happened to me...

I should also add that you can do the same thing with the intake side of the motor. I have 2 filters for my truck, both K&N's 1 is the stock filter that came with the FIPK II kit and the other is one that I bought from summit (7x9" round). with the big filter, my low end goes down a bit (butt dyno can tell) and the top end goes up. easy to tell. with the small original filter, the low end goes up and the top end goes down.

I think the key is to find what makes you happy. I try to get the best performance all around... fuel economy, low end and high end.