Adam, I have an 03 Cummings Diesel 2500 automatic, long bed, 4dr,w 4.10 rear. Only thing I added was a K&N Filter and a bed cover. I'm Getting 19.1 MPG. You must have clogged fuel injectors, or the 02's are off. Something is AWRY.. You should be getting like 11-12 IMO. Try a bottle of fuel...