Is the 5.9 or 6.7 Cummins better?
For most buyers today, the 6.7 Cummins is the better choice due to stronger power, greater torque, and modern emissions technology, while the 5.9 Cummins remains appealing for those who value mechanical simplicity and potentially lower up-front maintenance costs.
To understand why, this article compares the two engines across performance, maintenance, emissions, and ownership considerations, and discusses who benefits most from each setup.
Background: A quick look at the two engines
What is the 5.9 Cummins?
The 5.9-liter Cummins has powered Ram pickup trucks for decades, evolving from the early 12-valve mechanical-era engines to later 24-valve, electronically controlled variants. It’s renowned for straightforward design, rugged reliability, and broad aftermarket support. Depending on the year, it lives in models ranging from lighter-duty duties to heavy-haul applications, and it generally features simpler fuel systems and fewer emission-control components than newer diesel engines. For owners who prioritize ease of maintenance, cheap used parts, and a proven track record, the 5.9 remains a compelling option.
What is the 6.7 Cummins?
The 6.7-liter Cummins is a newer, more technologically advanced inline-6 that replaced the earlier 5.9 in Ram heavy‑duty trucks starting around 2007. It uses modern common-rail fuel injection, more valves per cylinder, and advanced emissions equipment (including diesel particulate filtration and selective catalytic reduction with DEF). The result is noticeably higher power and torque, improved towing capability, and better performance on demanding duties—at the cost of greater complexity and higher potential maintenance expenses related to aftertreatment and sensors. For buyers who need maximum capability and current technology, the 6.7 is the standout option.
Performance and towing
Below is a practical comparison of how the two engines behave in common workloads. The details vary by year, spec, and tuning, but the general differences hold across generations.
- Power and torque: The 6.7 Cummins delivers substantially more horsepower and torque than the 5.9, giving quicker acceleration and stronger pulling power, especially under heavy load or when climbing grades.
- Towing capability: With its higher torque and modern electronics, the 6.7 typically yields superior towing performance and higher maximum trailer weights on newer models, while the 5.9 remains capable for many towing tasks, particularly in older trucks or lighter configurations.
- Fuel efficiency and driveability: The 6.7’s modern fuel system and emissions controls can offer better efficiency in real-world driving, especially when combined with newer transmissions and gearing. The 5.9 can be very efficient in proper setups, but older designs may lag in urban stop-and-go scenarios or under heavy load when compared to a well‑tuned 6.7.
- Maintenance of performance hardware: The 6.7’s turbo, intercooler, and common-rail system, plus aftertreatment, allow for more consistent performance, but also introduce more potential failure points than the simpler 5.9 configuration.
In everyday use, the 6.7’s power translates into confident highway merging and towing, while the 5.9 still delivers dependable performance with fewer moving parts to monitor.
Maintenance and ownership costs
Before weighing the numbers, consider how maintenance costs and ownership logistics differ between the two engines. The following points summarize typical realities owners encounter.
- Emissions equipment and DEF: The 6.7 relies on exhaust aftertreatment (DPF, DEF/SCR), which adds ongoing DEF refills, regeneration cycles, and potential aftertreatment repairs. The older 5.9 engines have simpler emissions setups, especially the 12V era, with fewer DEF-related maintenance items.
- Parts cost and availability: Both engines have strong aftermarket support, but genuine parts for the newer 6.7 can be more expensive, and labor costs may be higher due to added sensors, emissions plumbing, and more complex fuel systems.
- Maintenance intervals and complexity: The 6.7’s turbo, high-pressure fuel system, EGR/DPF, and related sensors create more potential failure points and diagnostic needs. The 5.9 is generally simpler to diagnose and repair, especially on older models with fewer electronic controls.
- Reliability and longevity: Both engines are built to last with proper maintenance. The 5.9 is often praised for long, trouble-free lifespans when well maintained, while the 6.7 has matured into a reliable platform, albeit with higher initial service costs tied to emissions hardware and newer design features.
For buyers prioritizing budget predictability and a simpler mechanical footprint, the 5.9 often edges ahead. For those who plan on heavy use, longer ownership, or needing current technology, the 6.7 tends to be the more practical choice despite higher upkeep.
Which engine is right for you?
If you own a classic Ram pickup or are working within a tight budget and want to avoid DEF/SCR systems, the 5.9 can be a compelling fit, particularly for tasks that don’t demand peak modern-torque. If you regularly tow heavy loads, need strong, consistent performance, and want the latest engine technology and emissions controls, the 6.7 is typically the better match—and it is the engine you’ll find in Ram’s current HD lineup.
Bottom line
In contemporary applications, the 6.7 Cummins offers superior power, torque, and modern efficiency, making it the preferred choice for most Ram HD buyers and owners who need high capability and long-term compliance with emissions standards. The 5.9 Cummins, however, remains a durable, simpler, and often more affordable option for enthusiasts, budget-conscious buyers, or those seeking a proven workhorse with fewer emissions-related complexities.
Summary: The 6.7 generally outperforms the 5.9 in power, towing, and modern technology, but the 5.9 delivers unmatched simplicity, lower up-front maintenance costs, and a well-proven reliability record. Your choice should hinge on how you plan to use the truck, your tolerance for emissions equipment, and your budget for maintenance and parts over time.
