YukonTorqueXL

Bought a used 2023 GMC Yukon XL SLT 3.0L Duramax in December 2024 with 7,900 on it. Absolutely love the truck and the diesel was the right choice as I'm seeing about 50% better fuel economy then a friend of mine who has a 2021 Suburban with the 5.3L. The vehicle did indicate the best MPG seen was 34.6 mpg when I bought it. Not sure how fast or what road someone was driving on but still impressive. My first 100 mile road trip saw 28 mpg avg and included mostly highway. We recently just took a longer road trip from outside of Philadelphia to Sunday River ME. Roughly 500 miles one way in very cold, wet and snowy conditions. Truck was loaded with 4 people, bags, food and Thule roof box for the skis. We avg 24 mpg and was all highway except for the last 60 miles. The computer indicated an additional 112 miles of range when we arrived. The roof box does give a 3-4 mpg penalty which I have seen in other vehicles of mine as well. Still given how big of a vehicle this is, I'm extremely happy with the result. I am running opti-lube XPD additive as well. On the way home the first 60 miles of the trip was more back roads to get to I-95 and we avg 28 mpg for that portion of the drive. The rest of the trip avg was 24 mpg. I do run in 2WD instead of AUTO as much as possible when road conditions allow. 95% of the trip was 2WD. I haven't truly tested if being in 2WD does provide better fuel economy or not over AUTO. Anyone else have a good read on this?