So I have a 2.5" catback exhaust and a header with a 2" collector on it. What size cat should I go with? Should I go with a 2.5" cat or a 2" cat? Or should I just split the diferrence and get a 2.25" cat so it gradually gets bigger from 2" to 2.5"?

I just want whatever will work best with what I already have.

 

Im gonna say 2.5" because it will cause less restriction than a 2"......will you notice a difference?.... probaly not