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I have installed headers & high flow muffler on my 2007 Yaris, In discussions with my engineer family and friends, after the fact, I have come to the conclusion that headers will have no affect on FE in the rpm range used by ecomodders [2000 rpm to 3000 rpm]. When the throttle is closed there will be an very small reduction in pumping loses of no use to anyone, at WOT and high rpm you will use more fuel than stock exhaust, hence more power... I would suggest that there is no good reason to take them off or put them on for FE.
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