WelcomeCustomer There is one big cause for this. The LB7 duramax used from 2001 to mid 2004 was 300HP, the LLY used from 2004 to early 2006 is 310HP The LBZ used 2006 and early 2007 has 360HP and The LMM used in new body style has 365HP and also meets the emissions standards. That requires some fuel to heat the exhaust filter to burn off the soot in the filter. BotXXXXX XXXXXne, more power, more fuel. I know some people have removed the filter and installed aftermarket programming and got better mileage, but the emissions will not pass anywhere. Have a great Friday afternoon and please ask if you need more information.
I think the ones that have worked are Diablo's.