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Well I noticed the increase in FE when the muffler really started leaking, granted this is all almost highway - high load conditions, ( very common for a non aerdynamic truck, ) you are putting out much more power than a car per mph on average, proof of this to me comes from the fact my sisters 88 Mustang 2.3 got over 12 mpg more than mine on a trip to fla, mine is the same year , also the more the throttle is open the more important free flowing exhaust would be , seems logical, my escort I just barely touch the throttle, the raanger another story, also ford upped the exhaust pipe and muffler sizes on the late 90's trucks, I had a 98 with the 4 and I noticed it was bigger, also got better fe, but the clincher to me was the immediate decrease in fe when I put the new muffler on, this was on the same almost daily 130 mile round trip , with very light traffic, so just relating my own experiences,
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