Replaced the cat today, it WAS extremly clogged, there was black **** pouring out of it when I took it off. Threw a 3" DP on there too while I was at it. BUT...my gas mileage still sucks at about 12mpg or less and the idle is still rough. I know I have polys but I've been in cars with polys before and it was nothing like this. I'm pretty much out of ideas as to what could be causing this. Only code is for the o2 because the DP doesn't have a bung. Someone suggested to me the other day it could be the tune...any ideas?