2027 GMC Sierra Gains V8 Power but Loses Fuel Economy

The 2027 GMC Sierra reverses a familiar new-model expectation: its larger V8 engines produce more power, but its reported EPA-estimated fuel economy is lower than that of the corresponding 2026 truck. In the clearest two-wheel-drive comparison, the rating falls to 15 mpg in the city and 19 mpg on the highway, from 16 mpg city and 21 mpg highway.

That is a loss of 1 mpg in city driving and 2 mpg on the highway. In return, the V8 fitted to most 2027 Sierras grows from 5.3 liters to 5.7 liters, while output increases from 355 to 402 horsepower. The result is a straightforward powertrain exchange: 47 additional horsepower accompanied by higher estimated fuel consumption over the same distance.

The four-wheel-drive Sierra follows the same pattern

The comparison also extends to the Sierra’s larger engine. A new 6.6-liter V8 replaces the 6.2-liter V8 offered for 2026. The 6.6-liter engine is standard on the 2027 Denali Ultimate and optional on some other higher trim levels.

With four-wheel drive, the 2027 Sierra is rated at 14 mpg city and 18 mpg highway. The comparable 2026 version was rated at 15 mpg city and 19 mpg highway. Both figures therefore decline by 1 mpg, showing that the mileage reduction is not confined to the more common 5.7-liter, two-wheel-drive configuration.

Those changes can look modest when expressed as one or two miles per gallon, but low baseline mileage makes each lost mile per gallon meaningful. At the two-wheel-drive highway ratings, a truck traveling 190 miles would use 10 gallons at 19 mpg, compared with about 9 gallons at 21 mpg. Actual consumption will vary with load, speed, terrain, weather and driving style, but the ratings indicate that owners would generally buy more fuel to cover the same distance.

Matched configurations are essential

Fuel-economy comparisons are useful only when the trucks are configured on a comparable basis. Two-wheel drive should be compared with two-wheel drive, and four-wheel drive with four-wheel drive. Mixing those layouts could wrongly assign a drivetrain-related difference to the model-year change or to an engine alone.

The same discipline applies to cab, trim, tires and other equipment, although the available figures here establish only the matched drivetrain comparisons. The two-wheel-drive numbers show a 2027-to-2026 decline from 16/21 mpg to 15/19 mpg, while the four-wheel-drive numbers move from 15/19 mpg to 14/18 mpg. These are the relevant pairs for evaluating the reported year-over-year change.

Engine displacement by itself also does not determine real-world fuel use. Transmission calibration, axle gearing, vehicle mass, aerodynamic drag, tire selection and operating conditions all affect consumption. For the 2027 Sierra, however, the reported ratings and engine changes point in the same direction: larger, more powerful V8s arrive with lower estimated mileage.

The 5.7-liter engine provides the most complete numerical view of what buyers receive for that loss. Its 402 horsepower nearly closes a 50-horsepower increase over the outgoing 355-horsepower 5.3-liter V8, but the corresponding two-wheel-drive truck gives up 1 mpg in the city and 2 mpg on the highway. The 6.6-liter option repeats the displacement increase and mileage decline, although no Sierra-specific output figure for that engine is established here.

For owners, the practical boundary is clear. The 2027 Sierra offers more V8 power, but it does not pair that gain with improved EPA-estimated mileage. At equal distance and fuel price, the lower ratings translate into more fuel purchased and a higher pump cost to use the newer truck.

By Robert McKinney — Editor-in-Chief for AMI’s automotive and mobility coverage, with a mechanical engineering background and a decade reporting on powertrain systems, EV innovation, and global vehicle manufacturing.