I had a 2015 Ecodiesel from new until traded in at 68K miles. The exhaust pipes never had any soot buildup and would pass a white glove test. The trade-in was to purchase a new 2017 Ecodiesel. When I wash this truck, I notice a light soot in the tailpipes. It has been this way from new and now has 41K miles. Swiping out the inside of the pipes where this thin layer of soot resides, the rag turns black. Nearly same trucks...2015 (27.7MPG on avg) had 3.55 rear, 2017 (24.5 MPG on avg) has 3.93. Same XL tires. This is not a complaint and the truck runs great but peaked my curiosity. Anyone ever take notice?