The 2027 GMC Sierra HD gets an all-new 8.3-liter Duramax diesel with 555 horsepower and 1,230 lb-ft of torque, an updated 6.6-liter gas V8 and new steering and braking hardware. Adaptive cruise control also becomes standard across the lineup, extending a useful feature beyond the luxury trims.
GM’s announcement fills in the powertrain details that were missing from the early order guide. Here is what changes for Sierra 2500 HD and 3500 HD buyers, from the engines to the standard equipment. The updated trucks are expected at U.S. dealers in late 2026; pricing will follow.
New Duramax and gas V8 specifications are confirmed
The 2027 Sierra 2500 HD highlights identify an L8S gasoline V8 and an L6P Duramax turbo-diesel V8. The outgoing L8T gas and L5P diesel codes appear in the deletions list. The 3500 HD engine-and-axle table also lists L8S and L6P.
The available 8.3-liter Duramax is rated at 555 horsepower and 1,230 lb-ft of torque, up from the outgoing diesel’s 470 horsepower and 975 lb-ft. The standard 6.6-liter gas V8 rises to 412 horsepower and 472 lb-ft. These are HD powertrains, separate from the engines announced for GM’s 1500-series pickups.
Allison ten-speeds, new steering and updated brakes
The 2500 HD engine-and-axle guide pairs the gas V8 with an Allison-branded 10-speed automatic and a 3.73 axle ratio. Diesel configurations list an Allison 10-speed and a 3.42 axle ratio, with a power-takeoff transmission option also shown.
GM adds rack-and-pinion electric power steering, electronic brake boost and 14.6-inch front brake rotors. Properly equipped 2027 GM HD trucks can tow up to 20,000 pounds conventionally or 38,000 pounds with a gooseneck. Those are lineup maximums, not ratings for every Sierra. Cab, bed, drivetrain, hitch configuration and the actual truck’s payload still matter.
Adaptive cruise becomes standard, with broader braking detection
Both HD highlights pages say adaptive cruise control is now standard on all trim levels. That makes the feature relevant to work-truck shoppers as well as Denali buyers. Adaptive cruise remains driver assistance, so drivers still need to supervise operation and follow the vehicle’s instructions.
GM also lists Front Pedestrian and Bicyclist Braking in place of Front Pedestrian Braking. The ordering change explicitly adds bicyclist detection to the feature name. That is not a claim that the truck can avoid every collision, and the equipment list does not provide an independent test of its performance.
Wheel and equipment changes round out the update
For the 2500 HD, GMC lists new standard 18-inch SLE/SLT wheels, 18-inch AT4X wheels and an LT265/70R18E all-season tire entry. Several previous 17-inch wheel and tire entries are deleted. Both 2500 and 3500 highlights also list a new brake code and removal of the tire-carrier lock.
The practical takeaway is to compare the complete 2027 truck, even if you know the outgoing Sierra HD. The engines, steering, braking and standard driver-assistance equipment all deserve attention. Confirm the selected wheel-and-tire combination and the actual truck’s payload and trailering labels before ordering. Final retail pricing is still pending, and equipment availability can change during the model year.
