Here are some baseline numbers based on what the EPA reported to start us off:
"The 2021 Chevy Tahoe returns 21 miles per gallon in the city and 28 mpg on the highway for a combined rating of 24 mpg for the rear-wheel-drive model. Four-wheel-drive diesel Tahoes see a slight drop, returning 20 mpg city, 26 mpg highway, and 22 mpg combined. The larger rear-wheel-drive Suburban suffers slightly, returning 21 mpg city, 27 mpg highway, and 23 mpg combined. However, the four-wheel-drive Suburban has the same fuel economy rating as the four-wheel-drive Tahoe.
The 3.0-liter Duramax first appeared in the 2020 Chevy Silverado, and it has the same output in the SUVs – 277 horsepower (206 kilowatts) and 460 pound-feet (623 Newton-meters) of torque. Chevy did tweak the mill for the Tahoe and Suburban, adding a new engine control module and reducing engine noise. The mill pairs with GM’s 10-speed automatic gearbox, with the Suburban and Tahoe offering 8,000 and 8,200 pounds of towing capacity, respectively, when equipped with two-wheel drive."