They're a real hit here in MT where we have no emission regulations and can modify our exhaust systems any way we see fit! Lot of them do delete kits for the new diesel trucks to get rid of EGR's, DEF and other restrictive emissions controls that rob horsepower and fuel mileage. Getting close to 30mpg is unheard of on a new Cummins until you delete all that junk and let her breathe.