Chevy's New Silverado V8s Reportedly Get 1–2 MPG Less Than the Engines They Replace
Documents posted by GM Authority put the new 5.7- and 6.6-liter V8s, and the carryover diesel, 1–2 mpg behind 2026 trucks. EPA listings aren't out yet.
Chevrolet's new 402-hp 5.7-liter and 481-hp 6.6-liter V8s make far more power than the engines they replace, but they reportedly give some of it back at the pump. GM documents posted by GM Authority and detailed by Jalopnik put the 2027 Silverado 1500's V8s 1 to 2 mpg below last year's 5.3- and 6.2-liter trucks, depending on configuration.
None of these numbers is final. The 2027 Silverado and GMC Sierra aren't listed on the EPA's fueleconomy.gov yet, GM hasn't commented publicly, and the figures could change during certification.
What the documents reportedly show
With two-wheel drive, the 5.7-liter is reportedly rated at 16 mpg city, 19 highway, and 17 combined. Adding four-wheel drive trims another 1 mpg from both the highway and combined figures. By Jalopnik's comparison, the 2WD truck sits 1 mpg behind the 2026 model in city and combined driving and 2 mpg behind on the highway.
The 6.6-liter, offered only with four-wheel drive, reportedly comes in at 14 city, 18 highway, and 15 combined, again 1 to 2 mpg lower than the outgoing engine. In the off-road ZR2, the same engine reportedly loses a further 1 mpg city, 3 mpg highway, and 1 mpg combined.
The carryover 3.0-liter Duramax diesel, still rated at 305 hp and 495 lb-ft, reportedly slips 1 to 2 mpg in most versions as well. The exceptions are city figures for some 4WD trucks and the diesel ZR2, whose city and highway numbers are reportedly unchanged. For reference, fueleconomy.gov lists the 2026 two-wheel-drive diesel Silverado at 23 mpg city, 28 highway, and 25 combined. In September, GM estimated a 2WD diesel 2027 truck at 27 mpg on the highway, the basis for its claim of more than 900 miles of range with the new optional 34-gallon tank.
Ratings for the updated 2.7-liter TurboMax four-cylinder, now at 350 hp, haven't been published. Separately, Autoblog, citing GM Authority, reports that diesel trucks get a much larger diesel exhaust fluid tank and lose the rear under-seat storage bins to make room for it and the bigger fuel tank.
Why a single mpg matters here
These are GM's highest-volume vehicles. The light-duty Silverado and Sierra together accounted for 297,461 US deliveries in the first half of 2026 alone, according to GM's sales data, and production of the new generation started in October.
The diesel slip may sting most. The Duramax is the efficiency pick of the lineup, and with diesel running more than $2 a gallon above regular, its mileage advantage is what makes the math work for many owners. A smaller advantage stretches the payback.
Jalopnik expects the V8 figures to apply to the GMC Sierra too, with the AT4X matching the ZR2, but no Sierra-specific numbers have surfaced. GM also hasn't said why the new engines would trail the old ones.
What comes next
The figures should firm up once the trucks appear on fueleconomy.gov, which would also reveal the 2.7-liter's numbers. We'll update this story when that happens. In the meantime, our 2027 Silverado 1500 price and trim guide and F-150 vs. Silverado 1500 comparison cover the rest of the picture.
