The 2027 GMC Sierra 1500 will carry a new 6.6-liter V8 rated at 481 hp, up from 420 hp for the 6.2-liter engine it replaces. Fuel economy numbers published on Nov. 6 show what the extra output costs: Denali and Denali Ultimate trucks with the larger V8 are listed at 14 mpg in the city, 18 mpg on the highway and 15 mpg combined.
Tech news worth reading: Get Techpresso, the daily email with the most interesting tech news and insights, read by 400,000+ professionals from Google, Apple and OpenAI.
We may earn a commission if you sign up.
The 2026 Sierra with the 6.2-liter V8 was rated 15/19/17, so the newer engine gives up two miles per gallon combined while adding 61 horsepower. A Sierra Denali ordered with the Duramax diesel goes much further on each gallon, and the gap is wide enough to change the math for anyone who tows or commutes long distances.
Motor1’s Brian Potter reported the figures on Nov. 6, working from GM Authority’s report on the new ratings, which in turn drew on General Motors’ 2027 fuel economy table. Motor1’s write-up frames the new engine as more powerful but less efficient, and the arithmetic supports that reading. The article does not attribute the numbers to a named GM spokesperson, so the published table itself is the evidence. Motor1 also lists a fourth engine, the 2.7-liter TurboMax four-cylinder, at 350 hp and 455 lb-ft with the same 10-speed automatic, which rounds out a four-engine Sierra range for the 2027 model year. The page does not carry any quotes from GM, and the page metadata describes the numbers as EPA ratings even though the article body never says so directly, a distinction worth keeping in mind until the agency’s own listings catch up.
481 hp and 501 lb-ft from the L78 block
GM detailed its sixth-generation small-block family before the ratings arrived, and AutoGuide’s spec rundown lists the L78 6.6-liter at 481 hp and 501 lb-ft of torque, compared with 420 hp and 460 lb-ft for the outgoing 6.2-liter. The shorter-stroke design borrows architecture from the Corvette’s 6.7-liter LS6 but trades peak track performance for durability and low-end pulling power.
The engine keeps the pushrod, cam-in-block layout with two valves per cylinder and adds a dual-fuel system that combines direct and port injection, plus a continuously variable oil pump. It pairs with a standard 10-speed automatic. For the Sierra 1500, GM quotes a maximum tow rating of 9,400 pounds and a maximum payload of 1,450 pounds with this engine, while the smaller 5.7-liter V8 is rated for 13,500 pounds of towing.
Denali, Denali Ultimate and AT4X ratings
The 15 mpg combined number applies to the Denali and Denali Ultimate. The AT4X off-road trim with the 6.6-liter V8 and standard tires is rated 13 city, 17 highway and 15 combined, and the same truck on mud-terrain tires drops to 13/16/14. Tire choice alone costs a full mile per gallon in that configuration.
Equipment World’s redesign coverage adds that the 6.6-liter is standard on the Denali Ultimate and optional on the Denali and AT4X, that GMC calls it its most powerful half-ton V8 ever, and that the redesigned Sierra is expected at dealerships before the end of 2026. The same report puts the Duramax at 305 hp and 495 lb-ft. The 6.6-liter is not the only new gas V8 in the lineup. A 5.7-liter V8 with 402 hp and 428 lb-ft replaces the 5.3-liter. It is rated 17 combined in two-wheel-drive Pro and Elevation trucks and 18 combined in four-wheel-drive Pro, Elevation, AT4 and Denali models. Motor1 points out that the 5.7-liter edges the 5.3-liter it replaces by one mpg combined in 4WD form, since the older engine was rated 16/20/17.
The result is a lineup in which the largest gas engine is the thirstiest. Buyers who choose the 6.6-liter are paying for towing and payload headroom they may not use daily, and for 481 hp that shows up in acceleration rather than at the pump. The comparison with the 5.7-liter is the cleanest one: for the same Denali nameplate, the smaller V8 returns 16/20/18 in four-wheel-drive form against 14/18/15 for the 6.6-liter, a three-mpg combined difference that works out to roughly 20 percent more fuel burned on the larger engine over the same distance. Motor1 says 6.6-liter trucks in the remaining trims post lower economy than their 5.7-liter equivalents but does not print those figures, so the Denali and AT4X rows are the only ones that can be checked side by side.
Duramax diesel and the efficiency gap
The carried-over 3.0-liter Duramax is the efficient pick. Rated 23/28/25 in two-wheel-drive Pro, Elevation and Denali trims and 22/26/24 in the four-wheel-drive versions, it returns roughly nine to ten more miles per gallon combined than the 6.6-liter in comparable Denali trucks. Motor1 says the diesel still beats the gas V8s at the same trim levels.
The gap is largest where the 6.6-liter is penalized most. The AT4X diesel is rated 21 mpg combined, six better than the gas AT4X on standard tires at 15. In two-wheel-drive form the diesel leads the 5.7-liter by eight mpg combined, 25 against 17.
Diesel trucks usually cost more to buy, and Motor1 does not publish 2027 pricing alongside the economy figures. The Drive’s spec download compares the new GM V8s with Ford and Ram rivals on horsepower, which is where the 6.6-liter makes its case. The unresolved question for shoppers is how ten mpg of difference over the first 100,000 miles stacks up against the diesel premium, a calculation GM has not yet published for the 2027 model year.
This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.
Tech news worth reading: Get Techpresso →
We may earn a commission if you sign up.
More from Morning Overview
- Ford is recalling 223,472 F-150 pickups because the fuel tank can leak or detach
- Hybrids have 15% fewer problems than gas cars, while EVs and plug-in hybrids have about 80% more, Consumer Reports finds
- Regulators cleared the first U.S. small modular reactor, 4 months early
- Lake Powell sank to a record 3,517 feet, nearing the level that stops Glen Canyon Dam’s turbines
