Is 5.9 or 6.7 Cummins better?
For most buyers, the 6.7 Cummins is the better pick for heavy towing and modern performance, while the 5.9 Cummins is a strong, simpler option for lighter use and budget builds.
The question hinges on how you plan to use the engine, your tolerance for newer technology, maintenance willingness, and budget. Below is a detailed comparison of design, performance, reliability, and ownership considerations to help you decide which Cummins fits your needs.
Design and Engineering at a Glance
Key differences in architecture and technology help explain why these two generations feel so different on the road and in the shop.
- Displacement and family lineage: 5.9L Cummins (the 6BT/Classic 5.9 family) is an older, simpler inline-6 design; 6.7L Cummins (ISB) is a newer evolution with more modern engineering and tighter tolerances.
- Fuel delivery and electronics: 5.9 engines rely on earlier fuel-delivery schemes with fewer sensors and, in earlier versions, mechanical or simpler electronic control; 6.7 engines use a modern common-rail system with more electronic control and diagnostics.
- Turbocharging and power delivery: 5.9s typically use a single turbo with setups tuned for reliability and mid-range torque; 6.7s employ more advanced turbo and tuning to deliver higher torque across a wider rpm range.
- Emissions systems: 6.7 engines include contemporary emissions controls (such as DEF/DPF) to meet current standards across the U.S.; many older 5.9s rely on earlier, less complex exhaust treatment.
- Mechanical complexity and maintenance footprint: the 6.7 is more capable but also more complex and service-intensive; the 5.9 is admired for its mechanical simplicity and straightforward maintenance.
In summary, the 6.7 brings modern technology, higher output, and better emissions compliance, at the cost of greater complexity and potentially higher maintenance expenses. The 5.9 offers robust, proven durability with simpler systems and lower upfront maintenance costs.
Performance and Towing Capabilities
Performance figures and towing capacity vary by model year and configuration, but general trends are clear: the 6.7 is significantly stronger and better suited to heavy-duty work, while the 5.9 is adequate for lighter tasks and classic builds.
- Power and torque: the 5.9 provides substantially less peak horsepower and torque than the 6.7, with ranges that reflect its older designs; the 6.7 delivers much higher torque across the rev range, improving hill climbs and heavy pulling.
- Towing capacity: the 6.7 commonly enables higher maximum towing limits on modern Ram 2500/3500 trucks, especially in configurations designed for heavy-duty use; the 5.9 is capable for lighter trailers but generally trails the 6.7 in max capacity.
- Fuel economy and driveability: daily driving with either engine can feel similar at light loads, but the 6.7’s greater torque and modern tuning tend to be more efficient under heavy loads; overall fuel economy depends heavily on gearing, weight, and payload.
- Reliability and maintenance costs: the 5.9 is often praised for its mechanical simplicity and lower repair costs, particularly in older builds; the 6.7, while very durable, can incur higher maintenance costs due to its advanced systems and DEF/DPF requirements.
Overall, the 6.7 is the better choice for regular heavy towing and modern-day performance, while the 5.9 remains appealing for budget-conscious buyers, classic restorations, or lighter-duty use.
Economics, Parts Availability, and Ownership Experience
Consider how much you’re willing to invest in maintenance, parts, and potential upgrades over time. These factors influence long-term ownership beyond raw performance numbers.
- Parts availability and cost: 5.9 parts are widely available and often cheaper due to its long production run and popularity in older trucks; 6.7 parts are plentiful but can carry higher price tags, especially for high-output variants.
- Service intervals and maintenance costs: the 6.7’s DEF/DPF system and related components add ongoing maintenance tasks and fluid costs; the 5.9 typically has lower ongoing fluid and emissions-service costs but may require more frequent attention to wear components as it ages.
- Resale value and market demand: modern 6.7-powered trucks generally command higher resale value due to capability and current emissions compliance; older 5.9-equipped vehicles remain desirable for classic- or budget-minded buyers.
In short, owning a 6.7 tends to be more premium and capable but pricier to maintain, whereas a 5.9 can be more economical and straightforward to repair, especially in older or lightly used applications.
Bottom Line: Which Should You Buy?
If you regularly tow heavy loads, work in trades that demand high payload and towing capacity, or want the latest emissions-compliant technology, the 6.7 Cummins is the clearer choice. If your priorities are simplicity, lower initial cost, and you’re working with lighter duty tasks or restoring a classic build, the 5.9 Cummins remains a compelling option.
Summary
The 6.7 Cummins generally offers more power, torque, and modern emissions compliance, making it superior for heavy-duty use and daily driving under load. The 5.9 Cummins excels in simplicity, ease of maintenance, and upfront cost, which can be appealing for builders on a tighter budget or enthusiasts restoring classic trucks. Your decision should hinge on intended use, willingness to handle newer technology, and total ownership costs rather than raw horsepower alone.
