After installation of bbk shorty headers what was your MPG as Mine is an average of 15 and burning drastilcy quicker. I’ve been running really rich And maybe a smell so im going to make sure everything is still tight before I buy the thicker metal gaskets but did the shorty headers improve gas Milage as it did not for mine.

 

I am getting 19 with mine mixed city and highway but I am also running a tune. It should not change your gas mileage much since it doesn't require a tune. Try unhooking your battery and see if relearing your idle and such.
BTW did you unhook anything by accident or like me unhook the right O2 sensor to drop the exhaust down to make more room?