Bought a 2015 duramax and decided to tune and delete the dpf for mpg purposes and a longer life. Also added an s&b cold air intake. Was getting great mileage of 16-17 in town for about 5 days then for whatever reason it just dropped to 14.5. My brother and friended noticed my tranny was acting lazy and used an edge insight or whatever and used tranny relearn and it’s up to 15 ish now but still. Anyone have any idea?