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I have the 4040xl....

Left the cat on and I can say that there is a slight increase of rumble at low throttle and you will notice a slight increase in depth idle

At highway speeds as soon as you reach your cruising rpm it does seem to quiet down to the point I think it is almost stock

As for increase in mpg...its just like any new toy, if you can keep your foot out of the skinny pedal you may see a .5 - .75 mpg gain....between the cool hose, muffler and K&N air filter I find myself listening to the sweet sound of lower mpgs..

BTW - when installing you will need a small / short piece of pipe to install inthe stocklocation...less than 6" I believe, local muffle shop welded in mine for under $75...couldn't get dirty for that price

my $.02
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