I dont know much about the early 99s. on mine (late 99 to 03) its 3.5 inches all the way back (including the DP) the 2.5 inches is the exhaust housing. I am not sure on the 2.5 inches, but I know its less then the DP size. If strokin early does not chime in, I would PM him, he knows alot about the early 99s.