The 66RFE and 68RFE are closely related six-speed automatics in Ram 2500 and 3500 heavy-duty trucks, with the same gear ratios. The 68RFE is the version built for the 6.7L Cummins diesel, with internals and a torque converter suited to diesel torque, while the 66RFE is used behind the gas Hemi V8s in heavy-duty Rams. They look very similar, so check the identification tag rather than appearance before ordering parts or a replacement.
66RFE vs 68RFE at a glance
| 66RFE | 68RFE | |
|---|---|---|
| Engines | Gas Hemi V8s in heavy-duty Rams | 6.7L Cummins diesel |
| Gear ratios | 3.23 / 1.84 / 1.41 / 1.00 / 0.82 / 0.63 | 3.23 / 1.84 / 1.41 / 1.00 / 0.82 / 0.63 |
| Internals and converter | Matched to gas engine torque | Matched to diesel torque |
| Fluid | ATF+4 | ATF+4 |
How to tell which one you have
The engine is the quickest clue: a gas Hemi heavy-duty Ram has the 66RFE and a Cummins truck with an automatic has the 68RFE, unless it is a high-output 3500 with the optional Aisin. The identification tag on the case and the truck’s build data confirm it. Parts such as solenoid packs and valve body components are often shared across the RFE family, but clutch packs, converters and calibrations can differ, so order by the exact transmission code and model year.
Can you put a 66RFE behind a Cummins?
No. Even where parts physically fit, the 66RFE is not built for diesel torque, and the bellhousing, converter and calibration are engine-specific. Replace like with like.
Reliability
Both share the RFE family’s design, and both can suffer from overheating with heavy towing, valve body and solenoid issues, and worn clutches; diesel torque makes these more common on the 68RFE, especially with added power. See 68RFE problems and transmission solenoid pack.
Keeping either healthy
- Use ATF+4 and service it more often if you tow.
- Use tow/haul mode when towing; see tow/haul mode.
- Keep the cooler clean and watch the transmission temperature on long grades.
Related comparisons
See 48RE vs 68RFE, 68RFE vs Aisin and 68RFE vs Allison 1000.
To confirm your code, check the build data or the identification tag on the transmission; see what transmission do I have?
Replacement units we list
- RoadReady Dodge 66RFE: $3,999.00
- StreetMonster Dodge 66RFE: $5,199.00
- RoadReady Dodge 68RFE: $4,299.00
- StreetMonster Dodge 68RFE: $4,899.00
Each includes a torque converter. Browse 66RFE transmissions and 68RFE transmissions.
More guides: 545RFE vs 68RFE · how long a 68RFE lasts.
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