24 MPG combined is an impressive figure for the 2020 GMC Sierra 4WD AT4 3.0L Duramax, especially considering what this truck is. The AT4 isn’t about chasing ultimate efficiency; it’s a lifestyle statement, a promise of off-road capability wrapped in a comfortable, tech-laden package, and the turbodiesel’s fuel economy lets you enjoy that lifestyle without constant gas station visits. This truck appeals to those who need real truck utility but prioritize comfort and a rugged image. They’re willing to pay for capability and features, and the Duramax’s MPG is a welcome bonus, not necessarily the primary reason for purchase. The AT4 buyer isn’t cross-shopping a Prius. They’re looking at rival off-road-oriented trucks and judging the total ownership experience.

City and highway performance

The EPA estimates the 2020 GMC Sierra 4WD AT4 3.0L Duramax at 22 MPG in the city and 26 MPG on the highway. These numbers reflect the efficient nature of a modern diesel engine, particularly when paired with a 10-speed automatic transmission like the one in the Sierra. The abundance of torque available at low RPMs allows the engine to loaf around town, minimizing fuel consumption during stop-and-go driving. On the open road, the Sierra’s aerodynamics and the transmission’s ability to select the optimal gear contribute to that 26 MPG highway rating, making long trips surprisingly economical for a full-size pickup. The Duramax is known for performing closer to its EPA estimates more consistently than gasoline engines because of its flatter torque curve and more predictable fuel consumption.

Annual fuel cost

The EPA estimates the annual fuel cost for the 2020 GMC Sierra 4WD AT4 3.0L Duramax at $2,300. This figure is based on driving 15,000 miles per year and using national average fuel prices. It’s a useful benchmark, but keep in mind that your actual fuel costs will vary depending on your driving habits, the type of driving you do (more city versus highway), and the price of diesel fuel in your area. Fluctuating fuel prices will make a significant impact on this number.

Real-world expectations

While the EPA estimates provide a solid baseline, real-world fuel economy often differs. Owners report that achieving the EPA’s 26 MPG highway rating is realistic on long trips, especially when driven conservatively. City MPG tends to be closer to the 20-22 MPG range. Towing or hauling cargo will significantly decrease fuel economy. Off-roading—a common use for the AT4–will also take a hit. The AT4 also has heavier off-road tires and a lifted suspension, both of which affect fuel efficiency. But even with these factors considered, the 3.0L Duramax generally delivers good MPG for a truck of this size and capability.

How it compares

Compared to gasoline-powered full-size trucks, the Sierra AT4 Duramax stands out. For instance, a 2020 Ford F-150 4×4 with the 3.5L EcoBoost V6 is EPA-rated at 19 MPG combined, making the Duramax a more efficient choice. Even Ram 1500 models don’t quite keep up with the Duramax. The Ram 1500 HFE EcoDiesel achieves a slightly better 26 MPG combined, but it’s a more basic work-truck trim, not the comfortable, off-road-ready AT4. If you’re interested in viewing all Gmc fuel economy data, or the SIERRA MPG across all years, we have resources available. If fuel economy is a key factor, the Sierra AT4 with the 3.0L Duramax offers an advantage over its gasoline-powered rivals in the off-road truck segment because of its better gas mileage.

Tips to maximize efficiency

The Duramax diesel is already fairly efficient, but small changes can make a difference. Avoid aggressive acceleration and hard braking; the engine excels at low-end torque, so use it to your advantage by accelerating smoothly and maintaining a steady speed. Anticipate traffic flow and adjust your speed accordingly to minimize unnecessary stops and starts.

Tire pressure plays a critical role in fuel economy. Ensure your tires are inflated to the recommended pressure, which can be found on the driver’s side doorjamb. Underinflated tires increase rolling resistance, which reduces fuel efficiency. Given the AT4’s off-road tires, ensuring proper inflation is even more critical.

Minimize aerodynamic drag. Remove any unnecessary items from the truck bed and consider using a tonneau cover to improve airflow over the vehicle. While the impact may be small, it helps, especially on the highway where aerodynamic drag is more pronounced. Avoid adding roof racks or other accessories that create significant wind resistance.

Service intervals are important to keep the Duramax running efficiently. Stick to the manufacturer’s recommended maintenance schedule for oil changes, filter replacements, and fuel system cleaning. A well-maintained engine operates more efficiently and delivers better fuel economy. Modern diesels are complex, so ensure you are taking your truck to a qualified technician.