Had 3" pipes with x-pipe and the larger 3" dynomax super turbo mufflers with 3" tail pipes to the bumper of my dart.. With the good 360 (low 12 sec. In the quarter.) Car was nice and quiet inside, no drones. Loved it.... Spun a bearing in it and put a stock 360 in it and it ran great with they big exhaust.... I wouldn't be scared of 3" exhaust with a stock motor especially if your upgrading down the road....

I have seen a couple articles saying like stated already. 3" to the muffler and 2.5" tail pipes.. It's supposed to keep the gasses flowing quickly. But who knows. Not sure if they take bends in the pipe in to account though... Not sure if anyone has done any real testing on it.. Be an interesting article for one of the magazines.