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the results shouldnt affect your gas mileage, however they should improve your throttle response, which may make your right foot heavier (this side effect may worsen your gas mileage)
I got a Spectre 3" flex pipe from oreilley auto today and 3" hose clamps. Everything was super easy to install and the 4.7 has a refined roar. All in all I only spent $25! I just loosened the hose clamps on the airbox, filter box, and TB and removed the hoses. I then removed the two screws that hold the airbox in place and took it all off. The rest is all easy enough to figure out.