I know that most in here drive a gasoline powered car and this won't really help your fuel economy, but for a diesel it will help improve things. Especially if you drive a turbo diesel.
I found this article where a guy reduces his engine intake losses by 1/2 a psi. (Note: part 3 is where it gets good)
http://autospeed.com/cms/article.html?&A=0629
That can reduce a regular diesel engines pumping losses quite a bit and for a turbo diesel it will improve the effectiveness of the turbo.