Actually the inner Milltek piping is 2.75.
I cut the 2.5 muffler tail pipe section off and had a 2.75 section welded on instead..
Did a thread on that a few weeks back.

 

I was reading up on what you had done. We saw that the inner pipes seemed a bit larger, but still not 3 inch. So instead of cutting and rewelding like you had done, I think we are going to go true 3 inch.

how did you determine that the inner piping is actually 2.75? how were you able to measure the inside?

 

When I bought it new and installed it, you could see the inner piping thru the connection pipe that joins the muffler that they were a lager O D then the 2.5 tail pipe. Plus I was told I was right by Milltek UK.