Okay, I have a 2.75" Tanabe downpipe and a 3" test tube with a 3" magnaflow exhaust. The exhaust is installed and has a 2.75" diameter right after the cat which goes to 3". After installing the test tube and downpipe I'm going to cut the piece of 2.75" pipe of the exhaust pipe and have a 3" welded on by a local exhaust shop so the it's 3" from the test pipe all the way through the exhaust pipe. Now, I don't see a problem connecting the 2.75" downpipe with the 3" test tube....shouldn't they bolt together even though the diameter is a little different?