not to stir any crap here but i did some very basic kind of measuring on a #17769 muffler where i work just a few days back and i can tell you for sure that its not 2½" internaly the measuring was done in a very simple way and not realy all that accurate but i would say about 2 3/4" internaly, unfortionaly that muffler is sold now but when they are on the shelf again i will have another look and do a better measuring that will be within 0,1mm,cant do any destructive check but can check the tubes that can be reached from the outside.

have in mind that a internal measurement of 2 3/4" isnt very far from the internal measurement of most 3" aluminized tubing that is measured O.D.
a simle formula in metric is 76.2mm od (3")tubing usualy has a wallthicknes of 2mm and there is two walls to count making it a total of 4mm of wall. 76.2-4=72.2 compared to 69.85mm (2 3/4") id measurement inside the muffler 72.2-69.85= 2,35mm

2.35mm of tubing thicknes is not a big loss in my opinion,its not right but its far from as bad as this thread may make it look, i bet there is bigger losses elsewhere in most any prebent exhaust system. ofcourse the loss gets bigger if you use thinner wall tubing. but there is probably bigger losses from the tubing being bent around small radiuses and at great angels.


edit sorry for all the metric numbers,im pretty comfortable with the imperial system but its just so much quicker and easier in métric for me to explain it in this case