2015 GMC Sierra 1500 SLE 31K miles. So I straight piped my exhaust and deleted the third cat now I get about 9 mpg literally went through half a tank in one day. Can anyone shoot me in the right direction as to why this is! My guess back pressure but I keep reading things that go back and forth about it. Long story short I just bought a new cat and borla atak full system to try and fix this issue. Running 93O