I currently own a 2020 6.6 , 2018 6.0 , 2009 Duramax and 2008 6.0 . The 2020 on factory 34”+ tires has the best fuel mileage non towing, the fastest, holds higher speeds on 7 % grades with the same trailer. The 09 is best towing mpg by 1 mpg , shifts the least in tow haul mode although the 2020 in man 5 ( same top gear as 09 in tow/haul) isn’t very far behind. 09 is the most comfortable to drive towing with cruise on in hills . 2018 moves loads from dead stop with the least amount of throttle, has much smaller tires 245’s is second with mpg non towing. The l5p 2020 will win them all over but the 6.6 gas is better than my 09 in my opinion