Five Diesel Engines With Better Gas Mileage Than The 6.6 Duramax

If you own a truck with the 6.6L Duramax diesel, you already know what it costs to feed it. The engine makes 470 horsepower and 975 lb-ft of torque, and it will haul just about anything you can legally put behind it — but it does not do any of that cheaply. Because the Silverado 2500HD and Sierra 2500HD (and most other heavy-duty trucks) exceed 8,500 pounds gross vehicle weight rating (GVWR), the EPA does not test them for fuel economy. 

The most honest answer available comes from Fuelly: Tracking 40 vehicles across 1,194 fill-ups and more than 402,000 miles of real-world driving put the 2025 Silverado 2500HD at 12.86 MPG combined, with a margin of error of ±0.17 MPG. At the more positive end, Car and Driver tested the 2026 Silverado 2500/3500 with the Duramax at 18 MPG. No matter how you look at it, the 6.6 Duramax Silverado is not one of the most fuel efficient trucks you can buy. 

For a lot of owners, that is the trade-off they signed up for. Still, not every diesel buyer actually needs what the 6.6L offers, meaning that they can enjoy adequate diesel performance without the impact on efficiency. If your heaviest tow is a camper, a boat, or a work trailer — and not a 20,000-pound gooseneck — there are diesel engines out there doing the same job for considerably less money at the pump. Here are five diesel engines with better gas mileage than the 6.6 Duramax.

If you still want a Silverado or a Sierra but don't want a diesel engine that drinks fuel with two straws, the 3.0L six-cylinder Duramax is right up your alley. Same badge, completely different conversation at the fuel pump. The 3.0L Duramax turbo-diesel in the half-ton Silverado 1500 makes 305 horsepower and 495 lb-ft of torque through a 10-speed automatic. According to the EPA, the 2WD configuration six-cylider is the most fuel-efficient non-EV Silverado you can buy, rated at 23 city / 28 highway / 25 combined MPG.

Four-wheel drive drops that to 22 city / 26 highway / 24 combined. Even in the ZR2 — a lifted, widened truck wearing aggressive tires — the rating holds at 20 city / 23 highway / 21 combined. In a Car and Driver instrumented test of a 2020 3-liter AWD Silverado 1500, the truck returned 26 MPG on their 75-mph highway loop and 23 MPG observed in real-world combined driving — matching its EPA city estimate exactly. If anything, that undersells what the engine can do in a less compromised configuration.

Cars tested a 2024 Silverado 1500 ZR2 — the least efficient 3-liter Duramax — and after refilling with 9.766 gallons at the end of a mixed suburban and highway loop, hand-calculated 21.3 MPG combined, almost exactly what EPA reported. The Duramax 3.0 is considered one of the best diesel truck engines because of its torque curve, power, and reliability. However, considering just how well it does on fuel, efficiency could also be added to that list.

Ford only sold the 3-liter Powerstroke engine for four years before killing it, which is a shame — at launch, it was no small achievement at the pump. The 3.0L Power Stroke made 250 horsepower and 440 lb-ft of torque. A 2WD F-150 equipped with it earned EPA ratings of 22 city / 30 highway / 25 combined MPG. The MotorTrend 2018 first drive noted this made it "the highest EPA-estimated ratings available in a fullsize, light-duty, two-wheel-drive pickup truck."

Owner results backed those numbers up: One documented 29.1 MPG hand-calculated over a 5,016-mile trip at 72–74 mph, and another recorded 26.2 MPG on a 200-mile highway run during break-in. Even with such efficiency, Ford pulled the plug after 2021 and replaced the Power Stroke with The PowerBoost hybrid, which was rated 24 MPG combined in 4WD. It was also priced $500 less than the diesel, and offered 600 more pounds of towing capacity. Used examples from 2018 through 2021 are still out there. 

For anyone who wants a diesel F-150 and covers a lot of highway miles, they are worth hunting down. When we compared Chevy and Ford 3-liter diesel engines, the Duramax came out ahead on power, torque, and longevity, but the Power Stroke was still praised for its fuel economy and smoothness. Either way, both the 3-liter Duramax and the 3-liter Power Stroke are considerably more fuel efficient compared to the 6.6 Duramax.

Some might say that comparing a 6.6 Duramax against two six-cylinder diesels inherently means better fuel efficiency, but what about an engine that is even bigger in size than the Duramax, yet still more efficient? The 2025 Ford F-250 comes with two versions of the 6.7L Power Stroke: A standard output at 475 horsepower and 1,050 lb-ft of torque, and a high output at 500 horsepower and 1,200 lb-ft.

When we wanted to know what's the MPG for a Ford 6.7 Power Stroke, owners reported between 17 and 20 MPG combined city and highway, although, understandably, how the truck is used has a huge effect on gas mileage. This is in line with what Car and Driver got via a standardized 75-mph highway test of the 2024 F-250 model that returned 20 MPG. However, when putting these engines under heavy-duty towing conditions, the gap between these two narrows sharply.

On the TFLTruck Ike Gauntlet — a maximum-weight tow up a 7% grade at altitude — the 2025 F-250 Tremor with the high output Power Stroke recorded 4.8 MPG versus 4.6 MPG for the Chevy 2500. Still better, but marginally. However, as we've seen with previous ratings during normal driving, at a stop light on the way home empty, the Power Stroke pulls well ahead of the 6.6 Duramax.

If you've read through the TFLTruck's comparison of the 6.7 Power Stroke and the 6.6 Duramax, you've probably realized that the 6.7 Cummins was also part of that comparison — and it won. The Cummins inline-six has been in the Ram heavy-duty lineup since 1989, and the 2025 version is the most significant update it has received in years. Ram replaced some bits, namely the transmission — swapping the old six-speed for a ZF eight-speed automatic. The engine now makes 430 horsepower and 1,075 lb-ft of torque across all trims.

When we compared the 6.7 Cummins against the 6.6 Duramax, the two engines were closely matched on fuel economy. However, when looking at independent fuel economy testing, the results for the 6.7 Cummins seem fairly strong. A January 2026 review of a 2025 Ram 2500 Laramie 4x4 by American Cars and Racing recorded 25 MPG on the highway. As mentioned in the intro, the 6.6 Duramax Fuelly numbers were rated at 12.86 MPG combined.

For the 6.7 Cummins, Fuelly notes that, across 26 tracked vehicles, 667 fill-ups, and 209,365 miles of real-world driving, the 2025 Ram 2500 comes in at 14.06 MPG combined — 1.2 MPG better than the 6.6 Duramax on the same platform. Every one of those numbers sits above what the 6.6L Duramax returns under comparable conditions. Granted, those differences are small; in some instances the Cummins is bound to be less or as efficient than the Duramax.

To finish our round-up, we have to mention the 3-liter EcoDiesel V6. Sure, it is a six-cylinder, and that makes it likely to be more fuel efficient than the Duramax 6.6 eight cylinder. However, not many expected the EcoDiesel to live up to its name as much as it did. Ram discontinued the EcoDiesel after the 2023 model year, but some of its MPG ratings are downright impressive. At its most efficient — a 2WD Ram 1500 — the EcoDiesel earned EPA ratings of 22 city / 32 highway / 26 combined MPG.

That 32 MPG highway figure is worth dwelling on. Most truck owners are covering highway miles, and 32 MPG is the kind of number that belongs on a midsize sedan, not a full-size pickup. To put that in context, the competing 3.0L Power Stroke topped out at 30 MPG highway and the 3.0L Duramax reached 29 MPG highway in their respective 2WD forms. All three light-duty diesels were running in a league of their own compared to anything the 6.6L Duramax posts unloaded. 

When we looked at what kind of MPG the EcoDiesel engine gets, the third-generation engine in the 2022 Ram 1500 averaged 24.6 MPG in real-world data, pretty close to what EPA ratings suggest. The third-generation EcoDiesel from 2020 through 2023 is the one to buy. It made 260 horsepower and 480 lb-ft of torque, supported towing up to 12,560 pounds, and never had the emissions issues of the early cars.

Gas mileage is never a fixed number, and presenting it as such is incorrect. Driving habits, payload, terrain, weather, tire choice, axle ratio, and even altitude can push real-world MPG figures significantly in either direction. A truck that returns 20 MPG on a flat Colorado highway loop might return 14 MPG in stop-and-go traffic with a trailer attached. It can also return 3 MPG when maxing out its towing capacity on a steep on-ramp. That variability is real, and any honest fuel economy comparison has to account for it.

To answer which diesel engines have better gas mileage than the 6.6 Duramax, we pulled from as many credible data sources as we could find. That included real-world aggregate data from Fuelly, official EPA ratings, instrumented tests from Car and Driver and Motor Trend, independent highway loop testing from The Fast Lane Truck, road tests from Cars.com and American Cars and Racing, manufacturer press releases, and previous work from our own writers. 

Where a single source gave a number, we looked for corroboration. Where numbers contradicted each other, we noted it rather than picking the most favorable. No single test defines an engine's fuel economy. What this list reflects is the weight of available evidence pointing in the direction of certain engines being better on gas mileage than the 6.6 Duramax.