2.5 or 2.25 inch piping on custom cat back exhaust?

[jl2fast4u]

Which one is better and why? Or does it really matter?

[Ugly Pete]

the smaller the better.

[2002GT_Celica]

For a GT-S since you're only going to be doing bolt-ons, 2.25" is probably the best bet. If you were doing internal all-motor work, or putting a turbo on the car, then you would want to go bigger. Going to 2.5" is going to give you a tad more power, MAYBE, but definitely make the car a whole lot louder.

[SuuPaSeDan]

consult the muffler, i have 2.5 since my exhaust used 2.5 piping, but the stock is only 2.25, its easier to increase the size at the flange than at the muffler