ok im really confused. i have a cat back sys on my 01 ram. got it off ebay it was a bolt on... it was advertised as a 2 1/4 inch exhaust system... now the way i understand it is ... Pipe is used to flow material such as liquid or gas.. Tube is used for structurual such as rollcages brushgaurds etc etc.. Pipe is measured from in inside diameter (ID) and Tube is measure from the outside diameter (OD) so i got some extra "pipe" that wasnt used for my kit (extenion peices for an extend cab long bed) and i measure it and the inner diameter is 2" and the outer is 2 1/4 leave 1/8 for the walls. so my understand is .. is that the exhaust on my truck right now is made out of tube not pipe... right? or am i way off and just smoked some hella ****

 

Which ever way you look at it tubing a pipe are the same thing. they can have different id and od depending on the thinkness of the wall. you have to look when you buy it. Like on exhaust it will list an outside pia and an inside. such examples would be exhaust tubing or drain pipes. exhaust tubing carries a gas and drain pipes carry a liquid.