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I have a 2000 XLT 4.6, get the same 13-14 mixed driving; maybe 15 all hwy. Has K&N filter, factory exhaust. I can't imagine how an alternator replacement could affect gas mileage? Are you manually checking the mileage, or going by an on-board computer? If by computer, perhaps varying voltage has made the computer work differently. I've never heard of a 5.4 getting 18 mpg.
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