2027 GMC Sierra V8s gain power but lose fuel economy to the Duramax diesel

The redesigned V8 engines in the 2027 GMC Sierra 1500 deliver more power than the units they replace but return worse fuel economy, and none of them matches the carryover 3.0-litre Duramax diesel at the same trim level, according to GM's 2027 fuel economy table.

The figures, reported by GM Authority from GM's own ratings, settle the trade-off buyers have been weighing since the new engines were announced: the diesel remains the efficiency leader across two-wheel-drive and four-wheel-drive configurations.

How the V8 ratings break down

The 5.7-litre L76 V8 in two-wheel-drive Pro and Elevation trims is rated at 15 mpg city, 19 mpg highway and 17 mpg combined. In four-wheel-drive Pro, Elevation, AT4 and Denali trims the same engine is listed at 16 mpg city, 20 mpg highway and 18 mpg combined.

The larger 6.6-litre V8 is thirstier again. Denali and Denali Ultimate trucks with that engine return 14 mpg city, 18 mpg highway and 15 mpg combined, while the off-road AT4X is rated at 13 mpg city, 17 mpg highway and 15 mpg combined. A separate AT4X configuration on mud-terrain tyres drops to 13 mpg city, 16 mpg highway and 14 mpg combined.

The diesel stays well ahead

At the same trim levels the Duramax tells a different story. Two-wheel-drive diesel trucks in Pro, Elevation and Denali trims reach 23 mpg city, 28 mpg highway and 25 mpg combined, and four-wheel-drive Duramax models in Pro, Elevation, AT4 and Denali trims manage 22 mpg city, 26 mpg highway and 24 mpg combined.

Even the diesel AT4X, usually the thirstiest trim in a truck range, is rated at 20 mpg city, 22 mpg highway and 21 mpg combined. That is six mpg better on the combined cycle than the petrol AT4X on standard all-terrain tyres, a gap wide enough to shift running costs for owners who tow or haul regularly.

That matters because the diesel is not a new engine fighting a new one on unequal footing. It is a carryover option beating the redesigned petrol line on the same test cycle, in the same trims, on GM's own numbers. For anyone cross-shopping the two, the fuel bill maths starts from a clear disadvantage for the V8.

More power, worse mileage than the engines they replace

The new 402-horsepower 5.7-litre V8 replaces the outgoing 5.3-litre, which made less power but returned better mileage: 16 mpg city, 20 mpg highway and 17 mpg combined in four-wheel-drive form for 2026. The new engine gains output yet still trails that combined figure.

The 481-horsepower 6.6-litre V8 follows the same pattern against the 6.2-litre it replaces. The outgoing 6.2-litre returned 15 mpg city, 19 mpg highway and 17 mpg combined, two mpg ahead of the new unit's 15 mpg combined rating in comparable Denali configurations.

GMC has already detailed the enhanced turbocharged 2.7-litre TurboMax four-cylinder as part of the 2027 Sierra lineup, rated at 350 horsepower and 455 pound-feet of torque. Every one of the four engines now uses a 10-speed automatic, so the efficiency differences come down to displacement, tuning and compression rather than gearing.

The pattern is consistent across every comparison: the redesigned V8s bring more power, return less mileage than the engines they replace, and leave the Duramax as the efficiency leader by a wide margin. Buyers weighing the 2027 Sierra's price and trim spread against real-world running costs now have the official numbers to work from.

What is still open

The fuel economy table covers the engine and trim combinations above; GM's ratings are the source for every figure here. Australian availability and pricing for the 2027 Sierra 1500 have not been announced, and the ratings quoted are in US mpg under the EPA system.

Volkswagen has capped charging at 80% on around 5,000 electric cars in the UK while it prepares a recall to check whether their battery components meet quality standards. The restriction is being applied by over-the-air update to selected ID 3, ID 4, ID 7, ID Buzz and Cupra Born models built between 2022 and 2025.

The company says the batteries, supplied by an unnamed supplier, contain components that might not meet its standards and could overheat when charged above 80% capacity, posing a fire risk. No injuries have been confirmed, although some property damage has been reported.

Which cars are affected and how many

The UK notice covers roughly 5,000 vehicles out of about 85,000 worldwide. A recall issued in other markets also lists Audi Q4 E-tron models built in the same period, but the SUV does not appear on the UK notice.

Volkswagen has warned that a failure could cause serious or life-threatening injuries to people inside or outside the car, as well as significant property damage. It says technical faults with the battery would not necessarily be recognisable, which is why every affected car needs to be checked.

What owners face in the meantime

Until their car is inspected, owners cannot use its full range. On official WLTP figures for the ID 3, the lost 20% could amount to between 40 and 70 miles of range.

Volkswagen says it has contacted retail customers where possible, advised fleet operators and posted notices on the relevant brand websites. Owners will be given a date and instructions in due course.

Some cars will need to go to a workshop for a physical safety check lasting one hour, with battery modules replaced where needed, while others can be checked and updated over the air. The company has not given a timeframe for completing the process.

The 80% cap is a blunt instrument, but it is the fastest way to cut the risk before any car has been inspected. For drivers, the practical cost is measured in miles of range lost on every charge, not in a repair bill.

Why the limit matters beyond these 5,000 cars

The restriction applies to models across several Volkswagen Group brands, which means the issue sits with a shared battery supply chain rather than a single model line. Audi Q4 E-tron owners outside the UK are covered by the wider recall, so the group's response varies by market.

Volkswagen has not named the battery supplier, and it has not said how many of the 85,000 cars worldwide will need physical checks rather than over-the-air updates. Those details will shape how long owners in the UK are stuck with reduced range.