Header/Exhaust Q's and recomendation for 01 yukon?

I have an 01 Yukon SLT 5.3 4wd. I'm looking to get a full exhaust for it. I was wondering, does it REALLY improve gas mileage or is that just a myth? If so, is it a noticable increase or is it just minimal? I'm already gettin about 18-19mpg avg on highway and 16-18mpg in town. So if it was more thatd be great.


Now, is there anythign I need to look for when choosing a good header and exhaust? I know i want side swept but thats really the only concern I have. Any recomendations on a good combo?

thanks alot

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