6.7 Cummins Vs. 6.6 Duramax Diesel: How Do The Two Engines Compare?

When it comes to big diesel engines for heavy duty trucks, you have a number of different engines at your disposal to get whatever job you need done. Modern turbo diesel engines are marvels of engineering that can produce nearly four-figures of torque no matter what brand you choose, and several exceed the 1,000 foot-pounds mark. Two immensely popular diesel engines are the 6.6-liter Duramax V8, seen in General Motors trucks like the GMC Sierra HD and the Chevy Silverado HD, and the world-renowned 6.7-liter Cummins inline-6 most prominently seen in heavy-duty Ram trucks.

It bears asking the question, how do the two engines compare? Aside from brand loyalty (which is still an important psychological factor), what makes someone buy a Duramax-powered truck over a Cummins-powered truck, and vice-versa? 

Given each brand's popularity, it's worth comparing the specs to see what makes each turbo-diesel tick (figuratively), and what each respective manufacturer touts as the ideal use case. By looking at what the manufacturer says about each engine and how the Duramax and Cummins compare, you can get a good bird's eye view on what might be the better option for you.

For a non-commercial truck buyer, the 6.6-liter Duramax turbo-diesel V8 is available on all current GMC Sierra 2500/3500 trucks and Chevy Silverado 2500/3500 trucks. Additionally, it can be equipped to chassis-cab frames for commercial trucks, and can be used to power anything from dump trucks to tow rigs. The 6.6-liter Duramax you will find in any 2025 or 2026 model year GM truck goes by the "L5P" internal designation, and was introduced in 2024 as an updated version of the older L5P-designated engines.

Specs-wise, it's a cast-iron block V8 with aluminum heads. It's stated to produce 470 horsepower, and 975 pound-feet of torque. As it stands, it's the most powerful diesel engine in any GM-made truck. According to GM, it was made to be quiet and efficient, those are definitely attributes that come in handy if you're hauling something a long distance. As such, publications like Car and Driver have seen upwards of 18 miles per gallon from Duramax-equipped Chevy trucks. Towing capacity is listed at 31,500 pounds for a properly equipped Chevy Silverado 3500.

The 6.7-liter Cummins might be one of the most recognizable diesel engines around, and quite possibly one of the most storied automotive engines ever. It's been in millions of Dodge and Ram trucks over the generations. The current iteration was released this year, but variations of the Cummins 6.7 can currently be found in anything from the Ram 2500/3500 trucks, to school buses; Some variations of the engine have even been adapted for marine use. It's a prolific powerplant with a huge aftermarket for 6.7 Cummins accessories.

The 6.7-liter Cummins can make a maximum of 430 horsepower and 1,075 pound-feet of torque, specifically from the "High-Output" specification. The maximum towing capacity for a Cummins-equipped vehicle is 36,610 pounds, as seen in the Ram 3500 with the right options.

For efficiency, the EPA doesn't actually certify most heavy duty trucks, so finding an official listed fuel economy rating will be a hard ask. However, dealers have said that you can expect between 16 and 18 miles per gallon while driving a diesel Ram 2500, depending on driving conditions and payload.

When it comes to numbers, the Cummins 6.7 is objectively more powerful than the Duramax 6.6. The fuel economy is roughly the same, though that will, of course, depend on what is being towed. Price, however, is where the engines differ even more: The least expensive Chevy to feature a Duramax will set you back $57,610 for a two-wheel drive, single cab, long bed 2025 Silverado 2500 WT. For a Cummins, the least expensive Ram featuring the engine is the single-cab long bed 2025 Ram 2500 Tradesman 4x4, priced at $61,185.

The Duramax is less expensive, and also a little less powerful than the Cummins. If you are only going for power, the Cummins is the easy choice. However, if you want economy and affordability, the Duramax is likely your best bet. Either way, you'll be able to tow a lot, and hearing the whistle of a turbocharger never gets old.