I had the Hipower off this weekend to fab a stainless straight pipe. I bought exhaust the car with the HKS on it so I've never noticed that the testpipe side of the midpipe is like 2.5"?? so, what size gasket do I need? The old one is done...like done. The test pipe is 3" but the exhaust is 2.5" at that connection.

2.5" gasket?

I know it seems like a dumb question but whatever..thanks

I would say get a 3" gasket.