Hey
When I went for NSW roadworthy last year I only had one muffler with a 4inch pipe at the back (how it was when I brought it). Very loud. They said I only needed to put something on to make it a bit quieter, a little resonator was enough to tone it down a little and pass roadworthy. There was nothing said that I needed a second muffle to pass, just make it quieter.
To be honest I am getting sick of the loud droning when doing cruising. Then the loud fuel pump. When I get the red back exhaust system installed i might put in an extra muffler just to make sure it is really quiet. Would this decrease fuel efficiency slightly though?
Cheers
Rob