EPA Fuel Economy: 18 MPG Combined

The 2024 GMC Sierra 2500HD with the 6.6L Diesel engine and 6-speed automatic transmission delivers a combined fuel economy rating of 18 MPG according to EPA estimates. This figure represents a balanced average between city and highway driving, making it a realistic benchmark for owners who use their heavy-duty truck for mixed driving conditions. For a truck in this class—designed to handle serious towing and payload capacity—the 18 MPG combined rating is competitive and reflects the efficiency improvements diesel engines offer over gasoline alternatives in heavy-duty applications.

Diesel engines like the 6.6L found in the Sierra 2500HD are engineered for torque delivery and durability, which naturally influences fuel consumption patterns. The 6-speed automatic transmission works in concert with the diesel powerplant to optimize gear selection across various driving speeds, helping the truck achieve respectable fuel economy despite its substantial size and towing capacity. For buyers prioritizing capability without completely sacrificing efficiency, this combination delivers a practical balance that many truck owners appreciate.

City vs Highway Performance

The EPA ratings for the 2024 GMC Sierra 2500HD show 16 MPG in city driving and 20 MPG on the highway. This 4 MPG difference is typical for heavy-duty trucks and reflects how these vehicles perform under different driving conditions. City driving—with frequent acceleration, braking, and idling—requires more fuel to maintain consistent performance with a truck of this weight and capability. Highway driving at steady speeds allows the diesel engine and automatic transmission to operate at optimal efficiency, where the 6.6L engine can maintain steady RPMs and burn fuel more economically.

If your driving consists primarily of highway miles, you can realistically expect fuel economy approaching the 20 MPG highway rating. Conversely, heavy city driving with frequent stops, starts, and short trips will see consumption closer to the 16 MPG city figure. Most owners experience fuel economy somewhere between these two numbers, depending on their specific driving patterns, terrain, and load conditions. Towing and hauling significantly impact real-world numbers and will reduce efficiency below the EPA combined rating.

Annual Fuel Cost

Based on EPA estimates, calculating annual fuel costs for the 2024 GMC Sierra 2500HD is straightforward. Using the 18 MPG combined rating, 15,000 annual miles, and a diesel fuel price of $3.50 per gallon, here’s the math:

Annual fuel cost calculation: 15,000 miles ÷ 18 MPG = 833 gallons of diesel per year. 833 gallons × $3.50 per gallon = $2,916 annual fuel cost. This translates to approximately $243 per month in fuel expenses at current pricing. Keep in mind that actual fuel costs will vary based on regional diesel prices, which fluctuate based on market conditions. Owners in areas with higher fuel prices can expect proportionally higher annual costs, while those in regions with lower diesel prices may spend somewhat less.

Real-World MPG Expectations

EPA estimates provide a reliable baseline, but real-world fuel economy for the 2024 GMC Sierra 2500HD typically ranges from 15 to 19 MPG depending on driving habits and conditions. Drivers who maintain steady speeds, avoid excessive idling, and drive primarily on highways often report numbers at or slightly above the EPA combined rating. Those who frequently tow, carry heavy loads, drive in mountainous terrain, or have aggressive driving habits should expect figures in the 13 to 15 MPG range.

To maximize efficiency with the 6.6L diesel engine, maintain proper tire pressure, keep up with scheduled maintenance including diesel fuel filter changes, avoid unnecessary idling, and plan routes to minimize stop-and-go driving. The diesel engine’s efficiency improves once fully warmed up, so combining multiple shorter trips into one longer journey can improve overall MPG. Using cruise control on highways helps maintain consistent speed and reduces fuel consumption compared to manual throttle control.

How It Compares

The 2024 GMC Sierra 2500HD’s 18 MPG combined rating compares favorably within the heavy-duty diesel truck segment. The 2024 Ford F-250 Super Duty with a 6.7L PowerStroke diesel achieves approximately 17 MPG combined, while the 2024 Ram 2500 with the 6.7L Cummins diesel delivers around 17 MPG combined as well. The Chevrolet Silverado 2500HD, which shares GMC’s diesel platform, achieves similar 18 MPG combined figures. These comparisons show that the Sierra 2500HD is competitive with its direct rivals in the heavy-duty truck market, offering comparable efficiency despite the substantial power and capability these vehicles provide.

Tips to Maximize Efficiency

• Maintain proper tire pressure: Underinflated tires increase rolling resistance and reduce MPG. Check tire pressure monthly and inflate to the manufacturer’s recommended PSI found on the driver’s door jamb.
• Use premium diesel fuel: High-quality diesel with proper detergents helps the 6.6L engine run cleaner and more efficiently than lower-grade options.
• Keep the engine properly serviced: Regular oil changes, fuel filter replacements, and engine maintenance ensure the diesel engine operates at peak efficiency.
• Minimize idling: Extended idling wastes fuel without producing miles. Modern diesel engines start easily, so turning off the engine during extended stops improves overall fuel economy.
• Reduce excess weight: Remove unnecessary items from the truck bed and cabin. Every 100 pounds of unnecessary weight reduces fuel economy, particularly in city driving.

Frequently Asked Questions

Is the 6.6L diesel better for fuel economy than gasoline engines in heavy-duty trucks? Yes. Diesel engines typically deliver 20-30% better fuel economy than comparable gasoline engines because diesel fuel contains more energy per gallon. The Sierra 2500HD’s diesel would achieve significantly lower MPG if equipped with a gasoline engine instead.

Will towing reduce the 18 MPG combined rating? Significantly. Towing loads can reduce fuel economy by 10-30% depending on trailer weight, aerodynamics, and road conditions. Expect 12-16 MPG when regularly towing with the Sierra 2500HD.

Does driving style affect the 18 MPG rating? Absolutely. Aggressive acceleration, excessive speeding, and frequent braking reduce efficiency. Smooth, steady driving can achieve MPG figures at or above the EPA estimate, while aggressive driving may see reductions of 2-3 MPG.

The 2024 GMC Sierra 2500HD with the 6.6L Diesel engine offers practical fuel economy for a truck built for serious work. With an 18 MPG combined rating and proper maintenance habits, owners can manage fuel costs effectively while enjoying the performance and capability this heavy-duty truck provides.