Added mass only hurts your fuel economy when you step on the brakes. Taking weight out will improve performance numbers but won't help fuel economy on the highway especially if you use any Pulse and Glide. It may be a lot of trouble and expense for nothing if you don't have a stop and go commute. Diligent PnG can actually improve economy with added mass if it is part of an aero mod.
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Here is an interesting comment about increased economy despite adding 1,300 pounds in an aero trailer.
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"The highest mpg I've recorded with my truck was when pulling my gap-filled, full-boat-tail trailer.I was 1,300 lbs heavier than stock and recorded 22% better mpg than the best ever recorded without it.(47.9 vs 39 mpg)"
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http://ecomodder.com/forum/323828-post14.html
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