2027 GMC Sierra EPA Ratings Are In: The 5.7L Holds Steady, but the Diesel Slips
This morning, we broke down the official 2027 Silverado EPA ratings. Now it’s GMC’s turn. The 2027 GMC Sierra EPA ratings are live on fueleconomy.gov, covering the new 5.7-liter V8, the new 6.6-liter V8, and the 3.0-liter Duramax diesel.
Since the Sierra shares its powertrains with the Silverado, you might expect identical numbers. Surprisingly, that’s not quite the case. In fact, the Sierra’s 5.7 holds steady against last year’s 5.3 in 4WD, while its diesel takes a bigger hit than the Chevy’s.
2027 GMC Sierra EPA Ratings vs. 2026 at a Glance
Here’s how each 2027 configuration compares to the closest 2026 equivalent. Specifically, that means the new 5.7-liter V8 against the outgoing 5.3-liter V8, and the new 6.6-liter V8 against the 6.2-liter V8 it replaces.
| Engine and Setup | 2026 MPG (City/Hwy/Combined) | 2027 MPG (City/Hwy/Combined) | Change in Combined MPG (%) |
|---|---|---|---|
| 5.3L V8 vs. 5.7L V8, 2WD | 16/20/17 | 15/19/17 | No change |
| 5.3L V8 vs. 5.7L V8, 4WD | 15/18/16 | 15/18/16 | No change |
| 5.3L V8 vs. 5.7L V8, 4WD with mud-terrain tires | 14/17/16 | 14/16/14 | Down 12.5% |
| 6.2L V8 vs. 6.6L V8, 4WD | 15/19/16 | 14/18/15 | Down 6.3% |
| 6.2L V8 vs. 6.6L V8, AT4X | 14/16/15 | 13/15/14 | Down 6.7% |
| 3.0L Duramax diesel, 2WD | 23/28/25 | 22/27/24 | Down 4.0% |
| 3.0L Duramax diesel, 4WD | 22/26/24 | 20/23/21 | Down 12.5% |
| 3.0L Duramax diesel, 4WD with mud-terrain tires | 21/24/23 | 20/22/20 | Down 13.0% |
| 3.0L Duramax diesel, AT4X | 19/20/19 | 20/22/21 | Up 10.5% |
For 2026, we used the standard listings rather than the flex-fuel 5.3. Also, the 2026 diesel 4WD, mud-terrain, and AT4X listings carry the EPA’s Sport Mode designation, since those were the only versions listed.
Still No 2027 EPA Ratings for the 2.7-Liter TurboMax
So far, the 2.7-liter TurboMax hasn’t posted official numbers for 2027. For reference, the 2026 Sierra version earned 20 MPG combined in 2WD and 18 in 4WD, so we’ll update you once its ratings arrive.
The 5.7-Liter V8 Holds Its Ground
Here’s the good news for Sierra shoppers. In 2WD, the new 5.7-liter V8 matches the outgoing 5.3 at 17 MPG combined. Better yet, in 4WD it matches the 2026 truck exactly at 15 city, 18 highway, and 16 combined.
That’s a better result than the Silverado, where the 4WD 5.7 lost one MPG combined. However, that’s mostly because the 2026 Sierra’s standard 4WD 5.3 listing was already one MPG lower than the Chevy’s. Either way, the 5.7 brings 402 horsepower and 428 lb-ft of torque, up 47 horsepower and 45 lb-ft, and it still runs on regular 87 octane.
How Sierra V8 MPG Has Changed Since 1999
To put the new numbers in perspective, we looked at the mainstream 5.3-liter V8 from the first model year of each Sierra generation. Since the EPA has updated older ratings to its current testing method, the numbers compare fairly across the years.
| First Year of Generation | V8 Engine | 2WD Combined MPG | Change From Previous Generation | 4WD Combined MPG | Change From Previous Generation |
|---|---|---|---|---|---|
| 1999 | 5.3L V8, 4-speed | 16 | Starting point | 14 | Starting point |
| 2007 | 5.3L V8, 4-speed | 17 | Up 6.3% | 16 | Up 14.3% |
| 2014 | 5.3L V8, 6-speed | 19 | Up 11.8% | 18 | Up 12.5% |
| 2019 | 5.3L V8, 8-speed | 19 | No change | 17 | Down 5.6% |
| 2027 | 5.7L V8, 10-speed | 17 | Down 10.5% | 16 | Down 5.9% |
Overall, the 2014 Sierra was the high point, with the EcoTec3 5.3 and a six-speed automatic reaching 19 MPG combined in 2WD and 18 in 4WD. After that, the 2019 Sierra 4WD slipped to 17, and the 2027 truck lands at 16. Still, today’s 4WD Sierra V8 beats the original 1999 truck by more than 14%, while making far more power.
Mud-Terrain Tires Take the Biggest Toll
Just like the Silverado, mud-terrain tires do the most damage. With the 5.7, a mud-terrain 4WD Sierra is rated at 14 MPG city, 16 MPG highway, and 14 MPG combined, down 12.5% from the 2026 5.3. Similarly, the mud-terrain diesel drops from 23 to 20 MPG combined, a 13.0% decline.
Consequently, if you want aggressive rubber on your Sierra, plan on making a few more fuel stops on long family road trips.
The 6.6-Liter V8 and the AT4X
The bigger 6.6-liter V8 replaces the 6.2-liter V8, and it still recommends premium fuel. For instance, a 4WD Sierra with the 6.6 is rated at 14 MPG city, 18 MPG highway, and 15 MPG combined, down from 16 combined for the 6.2.
Meanwhile, the 2027 Sierra AT4X with the 6.6 slips from 15 to 14 MPG combined. In return, the 6.6 makes 481 horsepower and 501 lb-ft of torque, compared to 420 horsepower and 460 lb-ft from the 6.2.
Diesel: A Drop for Most, a Gain for the AT4X
Surprisingly, the 3.0-liter Duramax diesel shows the biggest swings in the 2027 Sierra EPA ratings. In 2WD, it drops one MPG to 24 combined. In 4WD, however, it falls from 24 to 21 combined, a 12.5% decline that’s steeper than the Silverado’s.
On the other hand, the AT4X diesel bucks the trend. Its rating climbs from 19/20/19 to 20/22/21, a 10.5% improvement in combined MPG. As a result, the AT4X diesel now matches the standard 4WD diesel at 21 combined.
Our Take on the 2027 GMC Sierra EPA Ratings
For gas buyers, the 2027 Sierra EPA ratings are a pleasant surprise. After all, the 5.7 delivers a big bump in power with no penalty in combined MPG compared to last year’s standard 5.3 listings. That’s a great trade for a family truck that tows the camper on weekends and runs errands all week.
Diesel shoppers, though, should look closely. Unless you’re buying an AT4X, most diesel configurations give up some efficiency for 2027. We’ll be curious to see whether real-world driving tells the same story once we get behind the wheel.
More From Our 2027 Truck Coverage
In the meantime, check out how the 2027 Silverado compares and what GM’s engine exchange program means for new truck owners. Also, sign up for our newsletter using the form at the bottom of this page.
