The best fuel economy news for GM’s 2027 full-size pickups belongs to an $80,000-class off-roader with a diesel. Every other rated Silverado 1500 and Sierra 1500 configuration matches or trails the truck it replaces.
The EPA has posted figures for the refreshed trucks’ 5.7-liter V-8, 6.6-liter V-8, and 3.0-liter Duramax turbodiesel. According to data compiled by GM Authority, nearly every powertrain and trim combination comes in lower than its 2026 counterpart, typically by 1 to 3 mpg.
The lone exception is the GMC Sierra AT4X with the turbodiesel. It gains 1 mpg city and 2 mpg on both the highway and combined cycles, landing at 20/22/21.
The gasoline drop has an obvious explanation. GM retired the 5.3-liter V-8 for a new 5.7-liter small-block and swapped the 6.2-liter for a 6.6-liter.
More displacement means more horsepower and more torque. Physics sends the bill to the fuel tank.
A 2WD Silverado with the 5.7 now rates 15 city, 19 highway, 17 combined. That’s down 1, 2, and 1 mpg. Go to four-wheel drive and highway drops to 18, combined to 16.
Bolt on mud-terrain tires with a Z71 or Trail Boss and the 5.7 slides to 14/16/14, a 2 mpg combined loss. The Sierra’s mud-terrain 5.7 gets hit harder, shedding 3 mpg on both highway and combined.
The 6.6-liter tells the same story. A 4WD Silverado High Country rates 14/18/15, while the ZR2 sits at 13/15/14, a figure that wouldn’t have looked out of place on a full-size truck decades ago.
The diesel is where the numbers stop making easy sense.
The Duramax keeps its 3.0-liter displacement, so there’s no bigger engine to blame. Yet the 4WD Silverado diesel in WT, Custom, Z71, and High Country trims loses 3 mpg on the highway, falling to 21/23/22.
The equivalent Sierra diesel in Pro, Elevation, AT4, and Denali trims fares worse. It drops 2 mpg city and 3 mpg on both highway and combined, settling at 20/23/21. That’s the steepest decline in either lineup, on the engine buyers choose specifically to save fuel.
Rear-drive diesels hold up better. Both the Silverado and Sierra 2WD Duramax trucks rate 22/27/24, down a single mpg across the board.
The 2027 trucks also bring updated transmissions alongside the new V-8s and refreshed sheetmetal and cabins. Outside the AT4X, the ratings show no sign of those changes clawing back efficiency.
The result is a strange pecking order. A Sierra AT4X diesel built for rock trails now matches the combined rating of a Denali diesel built for pavement.
Trim and drivetrain choices now swing the numbers nearly as much as the engine itself. Some ZR2 and mud-tire diesel configurations held steady in city driving, while their street-tire siblings slipped.
By the EPA’s own math, the 2027 Silverado and Sierra are a step backward for most buyers. The new V-8s deliver more output. The diesel delivers the same displacement and, in most four-wheel-drive configurations, fewer miles per gallon.
One piece of the puzzle is still missing. The EPA hasn’t yet published figures for the base 2.7-liter turbo-four, the engine GM leans on as its efficiency entry point.
Until those numbers land, the only 2027 GM half-ton that actually got more efficient wears an AT4X badge and a set of off-road shocks.
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