A 2-3 mpg drop is not unusual, but just opening up the exhaust should not have dropped your mpg that much, for the most part doing a tuned exhaust shouldn't effect you mpg at all, but in rare cases it does. I'm going to say that you need to do a plug chop in your entire throttle band to be able to know how to optimize your jetting, needle position, A/F mixture. A throttle chop is about your best bet unless you have access to a Dyno meter, as a dyno doesn't lie and will let you dial in perfect. I over 65 you can look for fuel drop, at 75 a radical drop.
"A sin of omission is a sin we should have commited but didn't."
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