My 2024 3500 dually gets terrible milage compared to my 2020 at4, I tow a toy hauler alot it is 47 feet and weighs 22000 pounds, I can go through an entire tank of Def in 100 miles. I liked my 2020 so much I kept it. I average 12 towing the trailer on my 2020 and barley use any Def, I average 6 in my new truck and have to carry 4 boxes of Def with me whenever I go out. My dealer did an ecm update and it got better, but just went out last weekend and again used a full tank of Def with a tank of fuel. Something is definitely off.