The K&N intake was the first thing i did to my 04 FX4 F150 and i saw great increases in mpg. I think I do more highway driving than most at about 97% that may be why I've seen a larger increase going from 460 km's(285mi) a tank to arround 575 km's(357mi). I loved the responce and sound so the first couple tanks had worse mpg due to a heavy foot problem lol. I paid over 450$ for my K&N and it paid for itself in less than a year at 5 tanks every 2 weeks 3 years ago when fuel was at an absurd price( 150-160 a tank)
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04 FX4 OBX long tube headders, K&N intake, Magnaflow offroad catback, 325/65r18 BFG's(34.6/13/18), Platinum smoked tail lights, HI/LO HID black projector headlights, painted to match brush guard, 2" level kit
http://i1243.photobucket.com/albums/...t/viper023.jpg