The 5.9 Cummins and the 6.7 Cummins are both inline-six turbo diesels used in Dodge and Ram heavy-duty pickups. The 5.9 (359 cubic inches) ran from 1989 to 2007 in three main versions: the mechanical 12-valve, the electronic 24-valve and the common-rail 5.9. The 6.7 (408 cubic inches) replaced it for the 2007.5 model year with a larger bore and stroke, a variable geometry turbo, more power and factory emissions equipment. The 5.9 is simpler; the 6.7 makes more power and torque and is quieter and cleaner.
5.9 vs 6.7 Cummins at a glance
| 5.9 Cummins | 6.7 Cummins | |
|---|---|---|
| Years in Ram pickups | 1989 to 2007 | 2007.5 to present |
| Displacement | 5.9 L (359 cu in) | 6.7 L (408 cu in) |
| Bore x stroke | 4.02 x 4.72 in | 4.21 x 4.88 in |
| Fuel system | Mechanical pump (12-valve), electronic pump (24-valve), common rail (2003 to 2007) | High-pressure common rail |
| Turbo | Fixed geometry | Variable geometry; see 6.7 Cummins turbo |
| Emissions equipment | Minimal | Particulate filter and EGR from the start; later trucks added SCR with diesel exhaust fluid |
| Power | Up to 325 hp in late high-output common-rail trucks | 350 hp at launch, rising past 400 hp in later high-output versions |
Exact ratings vary by year and transmission; see 6.7 Cummins specs and Cummins generations.
Towing
The 6.7 has more torque, stronger exhaust braking from its variable geometry turbo and trucks built for higher tow ratings, so it is the stronger towing engine. A healthy 5.9 still tows well within its truck’s ratings; see exhaust brake and tow/haul mode.
Fuel economy
Many owners report that older 5.9 trucks return similar or better fuel economy than early 6.7 trucks, largely because of the 6.7’s particulate filter regeneration. Later 6.7 trucks with SCR improved. Real-world results depend heavily on gearing, load and driving.
Longevity and maintenance
Both engines are known for durability; see how long does a Cummins last? The 5.9’s common problems are covered in 5.9 Cummins problems, and the 6.7’s, including emissions system upkeep, in 6.7 Cummins problems.
Bore and stroke explained
The 6.7’s larger bore and longer stroke add displacement, which raises torque at low engine speeds. That is why the 6.7 feels stronger pulling a heavy trailer from a stop, even before its higher factory ratings. The 5.9 revs similarly and remains a favorite for simple, mechanical builds, especially the 12-valve; see what does a crankshaft do? for how stroke is set.
Which is right for you?
- Simplicity and a lower purchase price: a 5.9, especially a 12-valve or 24-valve.
- Heavy towing and a modern truck: a 6.7.
- Swapping into another vehicle: see Cummins swap and 12-valve vs 24-valve vs common rail.
Replacement engines
We list both: see 5.9 Cummins engines and 6.7 Cummins engines.
More guides: Cummins swap transmissions · 5.9 Cummins specs.
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