Message: The real restriction in the factory exhaust is the 1 1/2" entrances in the crappy factory Y-pipe. I installed a Magnaflow 2 1/2" dual-into-3" Y-pipe and a 3" Carsound hi-flow "third cat" on my '01 4.7, and the gas mileage improved a solid 1 mpg. Both items were $300 total, installed, at a local muffler shop. Since I drive the Dak 30K+ miles annually, the mpg savings will recover the cost in no time; quicker at $3/gal., of course.
Subject: RE: Catalytic converter