When is the mileage the worst? The C7 isn't the biggest engine out there and you may just be overworking it, giving you the bad results.
For example, I got better fuel mileage pulling a heavy trailer with a 454 equipped dually crewcab then I do pulling a smaller trailer with my new 5.3L 2010 Silverado.
This is why I put a C15 in my Freightliner Century, that is soon getting converted to a motorhome.
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