66RFE Gear Ratios and Specs: Ratios, Clutch Chart, Axle RPM and Quick Spec Sheet

Every Ram spec sheet that lists the 66RFE prints the same seven numbers: 3.23, 1.84, 1.41, 1.00, 0.82, 0.63 and 4.44 Reverse. Some parts catalogs and rebuild manuals print 1.83, 0.81 and 0.62 instead. Neither is wrong, and once you know why, you also know something useful about this transmission: its gearset is the same one Ram put behind the Cummins.

This page is the 66RFE reference I keep on the bench. It covers the exact ratios, which clutches make each gear, the axle ratios Ram paired with it from 2013 through 2018, the engine rpm those combinations give you on the highway, the crawl and reverse numbers, and how the controller uses the ratios to decide a clutch is slipping. Every spec comes from Ram's own spec sheets or a recognized transmission authority. The full list is at the bottom.

Recommended 66RFE parts

When a ratio code or slip test points inside the unit: a replacement 66RFE planetary for a worn gearset, the 2014-on clutch module for burned frictions, and the 2014-18 master kit for a full overhaul. Ships from the USA. Secure checkout.

66RFE Planetary

$202.99View part

66RFE Clutch Module 2014

$157.06View part

66RFE Master Rebuild Kit 2014-18

$684.11View part

66RFE Gear Ratios at a Glance

Ram's 2013, 2014 and 2018 Ram 2500 Heavy Duty spec sheets all list the 66RFE the same way: "Three planetary gear sets, one overrunning clutch, full electronic control, electronically controlled converter clutch," with these ratios:

Where do the three-decimal numbers come from? Raybestos Powertrain describes the 66RFE as a 545RFE case that "contains many of the same internals and gearset as the 68RFE." When Dodge launched the 68RFE behind the 6.7L Cummins for 2007, its press release gave that gearset to three places: 3.231, 1.837, 1.410, 1.0, 0.816 and 0.625, a 5.16:1 spread. Ram's spec sheets round those to two places, which turns 1.837 into 1.84. ATSG's clutch chart truncates instead, which turns it into 1.83, 0.816 into 0.81 and 0.625 into 0.62. Raybestos lists the 66RFE as 3.23, 1.83, 1.41, 1.00, 0.82 and 0.63. Same gears, different rounding. If two catalogs disagree in the second decimal, it is not two different transmissions.

The step column is our arithmetic from the exact ratios. The 1-2 shift is the largest rpm drop in the box, about 43 percent, and 4-5 and 5-6 are both around 1.3. On a gas HEMI that makes its peak torque at 4,000 rpm, that big 1-2 step is why a loaded 66RFE truck feels like it falls off the power after the first shift. It is gearing, not slip.

Which Clutches Make Each Ratio

The 66RFE gets six forward gears out of three simple planetary sets by applying two friction elements at a time, plus one freewheel, the low sprag. ATSG documents the clutch application for this gearset in its 68RFE manual, and because the 66RFE shares the gearset and internals, the chart carries straight across:

• Park / Neutral: low/reverse (L/R) clutch only.
• Reverse (4.44): reverse clutch plus L/R clutch.
• 1st (3.23): underdrive (UD) clutch with the low sprag holding. The L/R clutch is also on, but only while output shaft speed is below 150 rpm.
• 2nd (1.84): UD plus the 2nd (2C) clutch.
• 3rd (1.41): UD plus the 4th (4C) clutch.
• 4th (1.00): UD plus overdrive (OD). Direct drive.
• 5th (0.82): OD plus 4C.
• 6th (0.63): OD plus 2C.

Read it column by column and the diagnostic value jumps out. UD works in 1st through 4th. OD works in 4th through 6th. 2C is in 2nd and 6th, and 4C is in 3rd and 5th. So if a truck slips in 2nd and 6th but nowhere else, look hard at the 2C circuit. If it slips only in 5th and 6th, OD is the common element. That pairing logic is faster than any guesswork, and it is how I decide whether I am buying a clutch module or chasing a valve body leak.

ATSG also notes that the shift into 5th and 6th happens only after the unit has completed the shift into 4th, and on the 68RFE those upshifts are delayed below 40 F or above 240 F fluid temperature. A cold 66RFE that holds 4th for the first mile is doing what it was designed to do.

Limp-in gear

The hydraulic system alone gives Park, Reverse, Neutral and 4th. When the controller shuts off the transmission control relay, the truck starts in 4th, 1.00:1, which feels like launching in third. ATSG adds that trucks with Electronic Range Select do not get a 2nd-gear limp position, because there is no "2" on the lever. A HEMI truck that suddenly pulls away like a slug and will not shift has probably defaulted, so scan it before you assume a mechanical failure. Our 66RFE limp mode guide covers what sets it.

Axle Ratios Ram Paired With the 66RFE

The 66RFE never came with the diesel's 3.42 axle. The gas engines make their torque high in the rev range, so Ram geared them shorter:

• 2013 Ram 2500 (5.7L HEMI only): 3.73, 4.10 and 4.56. Ram's 2013 sheet lists overall top gear as 2.35 with 3.73, 2.58 with 4.10 and 2.87 with 4.56.
• 2014 Ram 2500 (5.7L and new 6.4L HEMI): 3.73 and 4.10.
• 2018 Ram 2500 (5.7L and 6.4L HEMI): 3.73 and 4.10.

Ram's ratings for the engines on the other end: the 5.7L at 383 hp at 5,600 rpm and 400 lb-ft at 4,000 rpm, and the 6.4L at 410 hp at 5,600 rpm and 429 lb-ft at 4,000 rpm, with a 5,800 rpm limit on both. Check the axle tag or the build sheet before trusting a ratio someone tells you, because 4.10 swaps are common on trucks that tow. The 66RFE identification guide shows how to confirm the transmission itself.

What the Ratios Mean at the Wheels

Overall ratio is gear times axle. Engine rpm with the converter clutch locked is mph × axle × gear × 336 ÷ tire diameter in inches. Using a 32-inch tire and the exact ratios:

Crawl ratio uses the 2.64:1 low range of the BorgWarner 44-46 and 44-47 part-time transfer cases, which all three spec sheets list for the HEMI 4x4s. Go up to a 35-inch tire and every rpm figure drops by about 9 percent. Have the tire size reprogrammed after the change, or the speedometer and the shift schedule will both be off.

Here is the part that matters for towing. A 3.73 truck in 6th at 65 mph turns about 1,600 rpm. The HEMI's torque peak is at 4,000 rpm. In 6th it is producing a fraction of its rated torque, and with a trailer behind it the converter clutch has to carry that load with no multiplication from the converter. That is why loaded 66RFE trucks hunt between 5th and 6th, and why the shift points get busy on grades. Tow/haul and a manual hold in 5th or 4th with Electronic Range Select keep the engine where it makes power and keep heat out of the converter and OD clutch. Our 66RFE towing guide goes into the cooling side.

Reverse is deeper than first

At 4.44, Reverse is 37 percent deeper than 1st. With 4.10 gears that is an 18.2:1 overall reverse ratio before low range. Backing a loaded trailer up a driveway is easier on this gearset than on most automatics, and the reverse clutch is not a common failure point as long as the L/R circuit is sealing.

How the Controller Uses the Ratios

The controller does not know which gear it is in. It calculates it. ATSG describes the input speed sensor reading a tone wheel on the input clutch housing and the output speed sensor reading the park gear teeth. The controller divides one by the other to get the actual gear ratio, and it uses the same numbers for speed ratio error detection and the clutch volume index it learns for each shift.

When the measured ratio does not match the commanded gear, it sets a ratio code: P0731 for 1st through P0736 for reverse. That gives you a simple scan tool check on any slip complaint. Log input rpm and output rpm, divide, and compare to the table. In 4th the two speeds should match. Under load in 2nd you should see about 1.84. If the number drifts high when the throttle goes down, the clutch pair for that gear is slipping, and the pairing list above tells you which clutches to suspect. If the ratio turns to nonsense or one speed drops to zero, look at the sensor and its wiring before you condemn the unit. Our 66RFE speed sensor guide covers that test.

66RFE Quick Spec Sheet

A few notes on that sheet. The 12-inch converter with an aluminum stator comes from ATRA's GEARS Magazine, which notes the 65RFE and 66RFE both use it. The line pressure range is Sonnax's figure for the RFE family, which includes the 66RFE: 60 psi minimum and 150-160 psi maximum at stall. A truck reading max pressure at idle in Drive has a pressure control problem, not a strong pump. Fluid is from Ram's owner's manual, which calls for ATF+4 only in the six-speed behind the gas engine and checks the level at 170-180 F. The clutch split is from Raybestos, which lists a 2012-2013 friction module (RGPZ-273) and a 2014-on module (RGPZ-272) with different overdrive frictions. Order clutch parts by year, not just by unit name. More on fluid in our 66RFE fluid guide.

FAQ

What are the 66RFE gear ratios?

Ram's spec sheets list 3.23 first, 1.84 second, 1.41 third, 1.00 fourth, 0.82 fifth, 0.63 sixth and 4.44 reverse. Some catalogs print 1.83, 0.81 and 0.62 because they truncate the exact ratios of 1.837, 0.816 and 0.625 instead of rounding them. It is the same gearset either way.

Does the 66RFE have the same gear ratios as the 68RFE?

Yes. Raybestos describes the 66RFE as a 545RFE case with many of the same internals and the same gearset as the 68RFE, and Ram's spec sheets list identical ratios for both. The difference is the axles: the 66RFE got 3.73 or 4.10 gears behind the HEMI, while the 68RFE got 3.42 in the 2500 behind the Cummins.

What rpm does a 66RFE truck run at 70 mph?

With the converter clutch locked in 6th on a 32-inch tire, about 1,710 rpm with 3.73 axles and about 1,880 rpm with 4.10s. Larger tires lower those numbers, and a tire size change should be reprogrammed so the shift schedule matches road speed.

Which clutches are applied in 6th gear on a 66RFE?

The overdrive clutch and the 2nd (2C) clutch. Fifth is overdrive plus the 4th (4C) clutch, and 4th is underdrive plus overdrive. A slip in both 2nd and 6th points at the 2C circuit, because 2C is the only element shared by those two gears.