I've got an '03 Ford Expedition XLT 4X4 with the 5.4 (3.73 differentials). I've been considering the addition of an open element air filter and intake and some type of cat-back exhaust to open it up and make it breath a little better. Anybody have any experience with these mods, and if so, what kind of power and milege improvements have you noticed. I'm currently running a K&N filter in the stock air box, and Mobile One synthetic in the crankcase. I am hitting almost 17 on the highway. I'd consider everything successful if I increased to 18 or better on the highway. I've thought about trying to have a true dual exhaust with a crossover fabricated, but I don't know if there is enough clearance with the independent rear suspension unless I go with side exits.