I haven't seen any difference. As stated above, it will be in 2wd mode until you lose traction, and will go back into 2wd mode once traction is regained. So unless you are losing traction a lot I wouldn't expect to see any difference in fuel economy., There is only a 1-2 mpg difference between a 4wd and 2wd, presumably due to the weight difference. I would wager that you will see that in all generations of the Duramax engine., In this guide, I’ll break down the fuel consumption of 4WD vehicles compared to their 2WD counterparts and share insights on getting the most out of your ride., Throughout this article, we’ll also discuss several reasons 4WD uses more fuel, how to use less gas in a four-wheel-drive vehicle, and whether or not AWD reduces your vehicle’s gas consumption. We’ll also analyze numerous differences between AWD and 4WD regarding their respective fuel economies., In my case, the drivetrain losses/weight of 4WD decrease gas mileage even if you don't use it. According to Fuelly, where I tracked this, my 2014 Silverado 4WD got an average of 15.9 mpg, while my 2015 Silverado RWD got 17.6 mpg., Learn how 4WD vehicles use more gas and the factors that impact fuel efficiency. Understand the role of engine RPM, transmission loss, and gearbox friction..