2027 GMC Sierra: V8 Engines More Potent, Less Fuel-Efficient Than Diesel
The 2027 GMC Sierra 1500 is set to hit the market with a revamped engine lineup, where new V8 powerplants promise enhanced performance but sacrifice fuel economy when compared to the returning 3.0-liter Duramax diesel. Official EPA figures reveal a clear trade-off: truck enthusiasts seeking raw power will find it in the redesigned V8s, but those prioritizing efficiency will gravitate towards the diesel option, which consistently outperforms its gasoline counterparts in miles per gallon across all two-wheel and four-wheel-drive configurations.
A closer look at the efficiency numbers highlights this disparity. The two-wheel-drive 5.7-liter V8 model achieves an estimated 17 mpg combined (15 city/19 highway). In stark contrast, the comparable two-wheel-drive Duramax diesel variant boasts an impressive 25 mpg combined. This substantial eight mpg difference translates into significant long-term savings for drivers who opt for the diesel, especially considering the operational costs associated with daily driving, towing, or hauling. The gap in fuel efficiency directly impacts the financial considerations for potential buyers comparing the new generation of V8 engines against the established six-cylinder diesel.
For example, the 5.7-liter L76 V8 in two-wheel-drive Pro and Elevation trims is rated at 15 city, 19 highway, and 17 combined mpg. When equipped with four-wheel-drive, across Pro, Elevation, AT4, and Denali trims, this engine yields 16 city, 20 highway, and 18 combined mpg. Moving to the larger 6.6-liter V8, Denali and Denali Ultimate models see 14 city, 18 highway, and 15 combined mpg. The off-road-focused AT4X with the 6.6-liter V8 achieves 13 city, 17 highway, and 15 combined mpg, with a slight dip to 13 city, 16 highway, and 14 combined for versions fitted with mud-terrain tires.
Conversely, the Duramax diesel presents a compelling case for efficiency. Two-wheel-drive diesel trucks in Pro, Elevation, and Denali trims achieve 23 city, 28 highway, and 25 combined mpg. Four-wheel-drive Duramax models, available in Pro, Elevation, AT4, and Denali, offer 22 city, 26 highway, and 24 combined mpg. Even the Duramax-powered AT4X, typically the least fuel-efficient variant in any truck lineup, manages 20 city, 22 highway, and 21 combined mpg. This represents a six mpg combined advantage over its gasoline-powered AT4X equivalent on standard all-terrain tires, a difference that could significantly influence the budgets of truck owners who frequently engage in demanding tasks.
Comparing the new V8s to their predecessors reveals a consistent trend. The new 402-horsepower 5.7-liter V8, which replaces the outgoing 5.3-liter unit, offers more power but falls short on efficiency. The 2026 5.3-liter V8 in four-wheel-drive configurations achieved 16 city, 20 highway, and 17 combined mpg, surpassing the new 5.7-liter's combined rating. Similarly, the 481-horsepower 6.6-liter V8, a replacement for the 6.2-liter engine, follows the same pattern. The previous 6.2-liter V8 delivered 15 city, 19 highway, and 17 combined mpg, which is two mpg better than the new 6.6-liter's 15 mpg combined rating in comparable Denali models. It's worth noting that all four engine options in the 2027 Sierra lineup are paired with a 10-speed automatic transmission, indicating that the observed differences in efficiency are primarily due to engine displacement, tuning, and compression ratios rather than gearing variations.
For prospective buyers navigating the 2027 Sierra's pricing and trim options alongside real-world operational expenses, the message is clear: the redesigned V8 engines deliver an increase in power but come with a reduction in fuel economy compared to the engines they succeed. The Duramax diesel engine, however, maintains its position as the undisputed leader in efficiency, offering a substantial advantage for those who prioritize lower running costs and reduced fuel consumption.