6.7 Cummins Vs. 6.6 Duramax: Here's How These Diesel Engines Compare

When looking at heavy-duty trucks from a few of the biggest industry players like RAM and Chevy, one deciding factor is which diesel engine is best for you. This choice may seem like a straightforward one — easily decided by comparing output figures. However, both of these engines provide certain advantages and pack plenty of performance punch, making the decision a bit more challenging. 

RAM 2500 offers the 6.7-liter Turbo Diesel Inline 6 from Cummins (standard version) that generates 370 horsepower and 850-pound feet of torque. There is also a high-output Cummins engine available for the RAM 3500 that boosts horsepower to 420 and offers a more substantial 1,075-pound feet of torque. In contrast, Chevy offers a 6.6-liter Duramax Turbo Diesel V8 on its Silverado HD models that produces 470 horsepower and torque at 975-pound feet. Both Cummins and Duramax made an appearance on our list of the most reliable diesel engines ever built, meaning regardless of which diesel engine you choose, you're getting solid and dependable power under the hood.

In terms of efficiency, the 6-cylinder Cummins (non-high output) offers better fuel economy than the larger V8 Duramax. The 6.7-liter Cummins also has a greater maintenance window, requiring fewer service trips than the comparable Duramax engine. However, the Duramax provides superior performance with a greater towing capacity and an Allison 10-speed automatic transmission versus the standard Cummins 6-speed automatic.

With 6-cylinders as opposed to the Duramax engine's 8, the Cummins is easier on your wallet in terms of trips to the pump. According to Fuelly.com, the 2023 RAM 2500 Cummins-equipped trucks showed an average of 15.6 mpg, whereas the Duramax achieved an average of 14.5 mpg overall.

It isn't just fewer cylinders that make the Cummins more efficient; its SCR or Selective Catalyst Reduction system provides 10% additional fuel efficiency, according to Cummins. Not only does this system help enhance miles per gallon figures, but it also contributes to cleaner tailpipe emissions and a greater span between required maintenance. With the 6.7-liter Cummins turbo diesel, instead of every 7,500 miles recommended for the Duramax engine oil change, the Cummins is double that at 15,000.

If you combine these factors with the lengthy history of Cummins dating back over a century and its lengthy stint with RAM trucks starting in the late 80s, the brand name does carry a bit more industry clout for some buyers when compared to the Duramax, which debuted more recently in 2001.

When comparing performance figures, the Duramax exceeds the standard 6.7-liter Cummins in terms of output. For example, with the Max Trailering Package and gooseneck hitch, the Duramax-powered Silverado 3500HD can pull up to 36,000 pounds. While no slouch, the standard Cummins under the hood of the RAM 3500 offers 22,660 pounds of towing capacity. Of course, at these immense pulling weights, these figures only matter if you're pulling something like heavy construction machinery.

Another benefit to the Duramax turbo diesel is that it comes fitted with an Allison 10-speed automatic transmission. Allison formed in 1915 and has been involved in a number of industries such as racing, and aircraft powerplants, but really made a name for themselves in the robust automatic transmission market. More than 250 manufacturers in both the military and commercial sectors utilize their products worldwide.

Beyond the notable brand of transmission, the Duramax also includes 10-speeds, which offers 4 additional gears over the Cummins. In terms of heavy-duty tasks, more gears allow an engine to remain longer in its optimum powerband, which keeps output in the sweet spot range of maximum torque production. It also can result in a smoother ride, which is something not found in every heavy-duty pickup. While the high-output version of the 6.7-liter Cummins edges out the 6.6-liter Duramax regarding torque, there are several easy performance upgrades for your diesel truck that can enhance output should the need arise.