Official fuel economy ratings for the 2027 GMC Sierra 1500 reveal that General Motors’ redesigned V8 engines trade efficiency for added power, leaving the returning 3.0-liter Duramax turbo-diesel as the pickup lineup’s clear mileage leader.
According to fuel economy documentation obtained by GM Authority, the new Gen 6 Small Block V8s yield lower combined mileage than the powertrains they replace while falling significantly behind the six-cylinder diesel. In two-wheel-drive Pro and Elevation trims, the new 402-horsepower, 5.7-liter L76 V8 achieves an estimated 15 mpg in the city, 19 mpg on the highway, and 17 mpg combined. Equipping the 5.7-liter engine with four-wheel drive across the Pro, Elevation, AT4, and Denali models shifts those figures to 16 city, 20 highway, and 18 combined.
The larger 481-horsepower, 6.6-liter V8 posted even steeper fuel consumption, earning ratings of 14 mpg city, 18 mpg highway, and 15 mpg combined in Denali and Denali Ultimate trims. That represents a two-mpg drop in combined driving compared to the outgoing 6.2-liter V8. In the off-road-oriented AT4X trim, the 6.6-liter drops to 13 mpg city, 17 mpg highway, and 15 mpg combined, falling further to 14 mpg combined when fitted with mud-terrain tires.
By contrast, the 3.0-liter Duramax diesel maintains a substantial advantage. Two-wheel-drive diesel configurations return 23 mpg city, 28 mpg highway, and 25 mpg combined, beating comparable 5.7-liter V8 trucks by eight mpg overall. Even four-wheel-drive Duramax variants register 22 mpg city, 26 mpg highway, and 24 mpg combined, while the diesel AT4X achieves 21 mpg combined—six mpg ahead of the gas-powered AT4X.
The Sierra’s powertrain lineup also includes an updated 2.7-liter TurboMax four-cylinder engine generating 350 horsepower and 455 pound-feet of torque. Across the entire 2027 Sierra family, all four engine offerings now route power through a standard 10-speed automatic transmission.
Source: motor1.com
