For what it's worth I recently picked up a one owner, 115,000 Km, 2004 2500HD 6L long box ext cab 4x4 with a 4.10 rear axle.
It runs between 17 and 18 L/100 combined city and highway. Dips to around 16.5 if I don't go into the city much. Looks like a newer one isn't much better.
In the same truck with a Duramax you can take 30% off of these numbers , sometimes more. My 2011 Duramax runs 12.5 - 13.0 in the same scenario. Dips into the 10's on the highway.
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