I have a 2006 sedan with the following mods:
MD ART Pipes
Plenum Spacer
Ztube
Muffler delete
I also have a Coupe midpipe laying around that I could toss on too.

I was told that I could be running lean and wasting extra gas by not getting a tune. Is this true? Should I be getting a tune?
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If you were running lean you'd be throwing error codes that wouldn't go away! My best advice is to get a complete catback Magnaflow exhaust for your sedan so you're not just making just noise...then you'll need a tune!
Gary

 

A tune makes a big difference, even on a bone stock G35.

 

+1 for full catback!

 

Absolutely get a tune. Mods in my sig, tune got me 16 hp and 22 lb torque and the torque gains were everywhere. Very noticeable difference after it was done.