My 2011 Silverado 2500HD CC LML averaged 15-17mpg with 70% / 30% mix of highway/city driving. My 2020 Silverado 3500HD CC L5P SRW is seeing an average of 17-19mpg with the same combination. Mileage is 788 so I'm optimistic this will improve even more once the engine is a bit more broken in.
Note: Average Highway speeds are around 65