I am a new diesel owner, bought a 2001 GMC K3500 Dually. I have added PPE programmer, S & B cold air kit, and 4" silverline turbo back exhaust. The truck just rolled 100,000 miles on it. I also use Stanadyne performance fuel additive. My mileage increased to around 17 or 18, now all of a sudden can't seem to get any better than 15. Any ideas?