I have a 2012 GMC 3500 crew cab long box with the 6.0, not sure of gear ratio. I use 16-17l/100km empty and 27-28l/100km with a 3000lb camper. I usually drive 100-110 km/h. When I tow the 2000lb boat with the camper on I still get 27-28l/100km. Not sure what I get when towing the boat with the box empty, I think it's around 23-24l/100km
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