i know there is a million posts on exhaust and have looked at some, but i am wondering if adding a flowmaster exhaust system to a 2000 4x4 xterra, will make a noticeable differnence in gas mileage?

Having done this a few months ago, I can tell you that you will not see any improvement in mileage until you get passed the cool sound it makes when you step on it.

after that you will see some improvement I see an additional 30 miles per tank on Highway.