Hey all new guy here and I just traded my old Frontier for a new CC 4x4 SV and have a couple of questions. I'm actually getting better mileage than I expected (16-17 mpg mixed) but I was wondering if you guys experienced slightly better mpg after break in? Also will it void any warranties if I have the stock muffler replaced with a Flowmaster 50? BTW I just want a little bit of rumble and not concerned about performance gains because I baby my vehicles anyways.