And all header tube/exhaust/muffler pipe is measured by OD so a 3" nominal pipe is <3" I/D or 3"-2 X the wall thickness. 3" ID would be a bit larger (maybe .15" or so) than 2 3/4" OD which is 70MM and a common European size and very hard to tell visually (at least at a glance) from 2 1/2....right FAST guys

Surely somebody has one on the shelf somewhere.