1999 2500 4*4 diesel untill a couple a months ago i was getting about 18-19 mpg uloaded 17 now. and17-18 mpg loaded now 15 with about 5000-6000 lbs also it does not seem to have the power it did have changed to a k&n air filter' changed fuel filter ' and ajusted the valves does anyone have any ideals thanks