The exhaust pipe on my truck is 1 1/4" I.D., which looks and seems right to me, but then I've never gone around measuring exhaust pipes. The reason I'm bringing this up is I've had several people tell me its too small, I'll have back pressure issues, etc. But these have been V8 people, and it seems that if my memory is any good, those old low compression sixes all had small exhausts. Can anyone back me up or enlighten me if I'm mistaken? (That's happened too!!) John