The 2027 GMC Sierra's V8 Is More Powerful, But Less Efficient

GM's own 2027 fuel economy table for the Sierra 1500's redesigned engine lineup finally shows how the new V8s stack up. The numbers settle a trade-off truck buyers have been asking about since the engines were announced: the returning 3.0-liter Duramax diesel still beats the gas V8s at the same trim levels.

The two-wheel-drive 5.7-liter V8 posts 15 mpg city, 19 mpg highway, and 17 mpg combined, while the diesel in comparable 2WD trims carries a 25 mpg combined rating. That gap changes the running costs for anyone cross-shopping GMC's new Gen 6 Small Block V8 engines against the carryover six-cylinder diesel.

Fuel Economy By Engine And Trim

2027 GMC Sierra 1500 AT4X

Photo by: GMC

According to GM Authority, citing GM's 2027 fuel economy table, the 5.7-liter L76 V8 in two-wheel-drive Pro and Elevation trims carries a 15/19/17 mpg city/highway/combined rating. In four-wheel-drive Pro, Elevation, AT4, and Denali trims with the 5.7-liter V8, the figure is 16 mpg city, 20 highway, and 18 combined. Equivalent 6.6-liter V8 trucks post lower economy.

Denali and Denali Ultimate trucks with the 6.6-liter V8 return 14 mpg city, 18 highway, and 15 combined. The off-road AT4X with the 6.6-liter is rated at 13 mpg city, 17 highway, and 15 combined, with a separate mud-terrain tire configuration listed at 13 mpg city, 16 highway, and 14 combined.

The Duramax tells a different story at the same trim levels. Two-wheel-drive diesel trucks in Pro, Elevation, and Denali trims hit 23 mpg city, 28 highway, and 25 combined. Four-wheel-drive Duramax models in Pro, Elevation, AT4, and Denali trims manage 22 mpg city, 26 highway, and 24 combined.

Even the diesel AT4X, typically the thirstiest trim in any truck lineup, is rated at 20 mpg city, 22 highway, and 21 combined.

That is six mpg better combined than the gas-powered AT4X on standard all-terrain tires, a gap wide enough to change the numbers for owners who tow or haul daily.

V8s Versus The Outgoing Engines

GM's New 5.7-Liter V8

Photo by: General Motors

The new 402-horsepower 5.7-liter V8 replaces the outgoing 5.3-liter, which made less power but returned better mileage: 16 mpg city, 20 highway, and 17 combined for 2026 in four-wheel-drive form. The new engine gains output but still trails that combined figure.

The 481-horsepower 6.6-liter V8 follows the same pattern against the 6.2-liter it replaces. The outgoing 6.2-liter returned 15 mpg city, 19 highway, and 17 mpg combined, two mpg ahead of the new unit's 15 mpg combined rating in comparable Denali configurations.

GMC has already detailed the enhanced turbocharged 2.7-liter TurboMax four-cylinder as part of the 2027 Sierra lineup. GM's own specifications put it at 350 horsepower and 455 pound-feet of torque paired with a 10-speed automatic transmission.

Every one of the four engines now uses a 10-speed automatic, so the efficiency differences comes down to displacement, tuning, and compression rather than gearing.

For buyers weighing the 2027 Sierra's price and trim spread against real-world running costs, the pattern holds across every comparison: more power from the redesigned V8s, less mileage than the engines they replace, and the Duramax still the efficiency leader by a wide margin.

More From GM:

GM Is Finally Bringing Hybrid Power To Three Of Its Largest SUVs

Over 535,000 GMC Sierra Trucks Investigated For Grille Parts Flying Off

GM’s New Mongoose 8.3-Liter Duramax Diesel Makes 555 HP, 1,230 LB-FT

GM's TurboMax Upgrade Gives Colorado And Canyon 350 HP

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