I just recently purchased a 2005 duramax 4x4 with about 65000mi. I believe that it is the 6.6l turbo. It has a few up grades like a k&n cold air intake and a banks monster muffler. It seem to be getting about 15mpg. And I don’t think that is very good but I am not sure. So I decide to run some lucus oil through to see if that may help but I have not run that tank out yet. Is there something else that I may need to check on the truck that could be causing the lack of mpgs? Or is this possibly as good as it gets?