Need Help with Exhuast on Yukon..

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I will never mess around with my Cats on my 2002 Yukon they work fine and i know the engine needs the back pressure from them to run correctly...

Heres my problem... I'd like to add a Thrush Glasspack where the stock exhuast is... i will still have my cats so it wont be as loud as they are on trucks without cats.. Heres my Question...Am i going to get worse gas mileage changing the exhaust? I dont care to get better I just dont want it to change... I'm gettin 17.5 in town...i dont care to mess around with it trying to make it get better.. I'm happy where it is... What i want to know is will puting a glasspack on my Yukon.. not a dual exhuast just a glasspack inplace of the stock muffler..will it hurt my gas mileage or cause any other long term problems? or if i install a 50 series Flowmaster muffler will that hurt my gas mileage at all? I like the sound of a glasspack.. But i dont want it to make my gas mileage worse...

Can anyone help me out here?