[1992MustangV8]

my tailpipe rotted so I want to replace the whole cat-back system (after h-pipe) with dynomax super turbo. I measured the inner diameter of the part that was rotted and it seemed 2.25" but the rotting might have messed it up so I can't say it's perfectly precise. Then I measured the H-pipe circumference and got 8" and did the math to find that diameter is 2.55". I don't know how thick the outer part of the pipe is, so I don't know if the h-pipe is 2.5" or 2.25" I'm pretty sure it's not stock, but I am guessing the mufflers/tail pipes are stock. (previous owner changed it, not me).

I am wondering what size muffler/tail pipe kit to buy and want the 2.5". Are you able to bolt-on a 2.5" to a 2.25" anyways? (that is the main question). Because I did out more math and think the h-pipe inner diameter is 2.33" (as long as the pipe outer width is same as that of the tail pipe.

So to make a long story short I just don't know what size to buy but would be much more confident about it if I knew you can bolt a 2.25 to a 2.5, but if not, how to I figure out what size I need? Thanks a lot

[EagleAutosports]

Stock pipe sizes are 2 1/4 inch diameter. If it's not a stock mid pipe, no telling what exact size it is, there are 2 1/4 inch aftermarket mid pipes but they're older versions. The newer ones seem to all be 2 1/2 inch.
If you want a 2 1/2 inch cat back exhaust, you can get one. You should be able to bolt it right up to the pipe you have now, combining 2 1/4 and 2 1/2 pipes is common, there shouldn't be any problems with it.
A picture of the pipes might be able to show what brand pipes you have.