According to AMs website its 2.25: https://www.americanmuscle.com/understanding-mustang-exhaust-systems.html#:~:text=2015%2D2018%20(S550)%3A%202.25%22
I'd highly recommend just doing the whole catback though. Ton of weight savings getting rid of the suitcase, and no need to piece parts together. It is much easier overall.
- 17SCMach1 5 years ago