I have a 2000 F-250 with a 6 spd and 4X4, I bought it about three weeks ago with 87,640 miles, I am at 89,100 now. The first tank i ran through it showed to be getting 17.2 mpg. While filling up with my second tank I installeed a K&N air filter. The second tank showed to get 18.6 and tonight Showed 20.53 mpg. No Tuner but the truck had a MPRB Exhaust when i bought it. This is my first diesel, but it seems to be just the opposite of my gas burner, seems the harder i run the diesel the better fuel mileage i get? Just wondering why?