Feeling a little dumb here, but how are exhaust pipe and mufflers measured? Does it go by the inside diameter or the outside diameter?
Any help would be appreciated.
Thanks!

 

pretty sure its ID...at least my piping measures 2.25" ID rather then OD

 

Actually it kind of depends. Mostly it is measured by ID, but if you are getting fittings, mufflers, etc. then you may need to deal with OD as well. For example, most exhaust tubing is 1/16" wall, so with 2" ID and a pair of 1/16" walls, the OD is 2-1/8". So if you want to slip a fitting over that pipe, you would need 2-1/8" ID on one end of the fitting and whatever size on the other end.