I have owned a 2002, 2004 and now 2008 3500 HD. Got 17 to 19 in first two. I have 72,000. on the 2008, got 17 when first purchased, now 12 to 14. When I take to dealer and they reflash I can get 19, this will las for a few tanks of fuel then back to 12 to 14. I can put my auto zone tester on and there will be no codes, I reset codes, even though there are no codes, and I go back to 18. I would like to put an edge cts and an exhaust brake on but would like to figure out what is wrong before I do anything. Dealer has no idea.