if you are concerned about mileage, you have the wrong vehicle, these usually get perhaps a bit better than 13.5, but their consumption rate and emissions, are right on the verge of being criminal. As far as I know the tailpipes on my truck have always been black, but I have only had it for 30 years, however once I install the same setup as you have (I'm in the process of polishing the intake, I sure wish I had spent the money on one already polished) I'll let you know, I live at sea level basically.
