I have a 2004, 3500. 6.0, 4 X 4, with 26K. I do mileage every time I fill up. Average 10-11.5 mpg. I've been going to the same gas station, and 2 times ago I noticed more power. Got it down 1/2 tank, filled it and got 15.7. Got it down to 3/4 tank, took 8 gal, and again 15.7. I'm stumped, but very happy. Does something change around 26K miles. I'm thinking they forgot to add the 10% ethel.