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I only have a little expirience and it's certainly not scientific. I went from a 1"7/8 to a 2"1/4 exhaust, cat back, on the ZX2 and didn't really see any noticable change in FE. Then changed my cracked cast iron log manifold to a nice SVT 4-2-1 header and didn't see any big changes in FE. During that time I was not tracking mpg numbers real close, just kinda observing miles and gallons used. However I did put in a catalytic converter where the orginal had been removed and the difference after many months of tracking FE was nil. During this whole time my FE has steadily improved from many other much cheaper mods. Like ChrisT, I believe with enough time and testing you could probably see a measurable benefit from tuning an exhaust to a certain rpm range, but the change would probably be very small unless coupled with a complete tuning of the system ie. intake tract, ports, valves, cams, and the electronic tune.
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