There's a guy I work with who has a 2005 KW ...Cat, I think, but I haven't looked under his hood. He welded the EGR shut, did some ECM stuff and he gets 8.5mpg average. That's according to the QC, so actual is probably closer to 7.75, but that's still impressive for a W9.It's a 07 truck with a 06 engine so no dpf. I just sold a 06 isx 500 I welded the egr shut on it and ran like a champ. It also had no mufflers.
My nissan altima was supposed to be the same way.The acert from 2004 to present is egr. It's stealth that most people don't know about. It's like a jake brake on the intake valve. As the piston comes back up after firing the exhaust valves open and the acuators(acert) opens the intake valves and lets exhaust go back into the intake. If you take off your air compressor hose or intake pipe your jaw will drop when you see the soot your engine is breathing.