65RFE to 66RFE swap for more towing and drivability

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Steve White

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So I have been contemplating doing the 65 RFE to 66 RFE conversion on my truck and am looking for some tips if anyone has actually done it or not. Programming wise it should be pretty easy as it was available on the 3500 dually with a 5.7 in 2014 as well. So with that being said I am going to be posting what the differences are between these transmissions as I am at the dealer and happen to have a spare 66 RFE laying around and am rebuilding currently.

First big difference between these transmissions that i have found is the gear ratios are completely different.
65 RFE 66RFE
1st 3.00:1 1st 3.23:1
2nd 1.67:1 2nd 1.83:1
2nd gear prime 1.50:1 3rd 1.41:1
3rd 1.00:1 4th 1.00:1
4th 0.75:1 5th 0.81:1
5th 0.67:1 6th 0.62:1
REV 3.00:1 REV 4.44:1

I have looked up in the parts catalog as I have that access (which is nice to have), There is no difference from the torque converter to the center section of the transmission. Everything that is different for these transmission is the same in the input clutch hub. Same Valve body, torque converter, and transmission pump. The biggest difference is when you get into the planetary's of the transmission. The 65 RFE has 5 pinion plaentary gears as the 66 has 6 pinion planetary's. there is two more clutch disc in the 4c clutch and the Low reverse also has two more clutch disc in them.

I know from experience that the week point of the RFE transmission is the input clutch assembly. This is where all the clutches burn up and I am almost positive that it is caused from either lack of maintenance or someone just not caring when there vehicle is overloaded. Anyway. I will post pictures later but was curious what everyone was thinking on this change.
 

Wild one

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So I have been contemplating doing the 65 RFE to 66 RFE conversion on my truck and am looking for some tips if anyone has actually done it or not. Programming wise it should be pretty easy as it was available on the 3500 dually with a 5.7 in 2014 as well. So with that being said I am going to be posting what the differences are between these transmissions as I am at the dealer and happen to have a spare 66 RFE laying around and am rebuilding currently.

First big difference between these transmissions that i have found is the gear ratios are completely different.
65 RFE 66RFE
1st 3.00:1 1st 3.23:1
2nd 1.67:1 2nd 1.83:1
2nd gear prime 1.50:1 3rd 1.41:1
3rd 1.00:1 4th 1.00:1
4th 0.75:1 5th 0.81:1
5th 0.67:1 6th 0.62:1
REV 3.00:1 REV 4.44:1

I have looked up in the parts catalog as I have that access (which is nice to have), There is no difference from the torque converter to the center section of the transmission. Everything that is different for these transmission is the same in the input clutch hub. Same Valve body, torque converter, and transmission pump. The biggest difference is when you get into the planetary's of the transmission. The 65 RFE has 5 pinion plaentary gears as the 66 has 6 pinion planetary's. there is two more clutch disc in the 4c clutch and the Low reverse also has two more clutch disc in them.

I know from experience that the week point of the RFE transmission is the input clutch assembly. This is where all the clutches burn up and I am almost positive that it is caused from either lack of maintenance or someone just not caring when there vehicle is overloaded. Anyway. I will post pictures later but was curious what everyone was thinking on this change.
Gave any thought to doing the 8 speed swap instead,even better gear ratio's ,and if you're towing,it's hard to beat the 8 speeds stump pulling 4.71:1 first gear,and closer gear ratio's.A lot less rpm drop between gear changes,and your 2014 could of been had with the 8 speed,so it makes it a pretty easy swap to.
 
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Steve White

Steve White

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The main reason I thought about doing it cause I have a free 66RFE that i only need to put about $300 bucks into.
 

Jeepwalker

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That's very interesting information. Do they look the same physically ...where it should just bolt in?
 

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This thread certainly has my attention. I’m guessing that the driveshaft is the same length provided the Tran is physically the same size.
 

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actually the 8hp75 8 speed in my '19 has 5.0 first gear. even better
 

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A guy could still pull stumps with the 66RFE ....just use 'reverse' LOL
 
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Steve White

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This setup is basically a direct bolt in as the transmission case, pump, valve body, and torque converter are all the same. The actual cased of the transmission is the same all the way back to the 45 and 545RFE transmissions from the 2000's era. I need to check the tailshaft of the transmission to see about for the transfer case.
 

Wild one

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actually the 8hp75 8 speed in my '19 has 5.0 first gear. even better
Nobody has been able to adapt the 75 in yet.There's been a couple guys who tried and Russell at Sound German hasn't been able to make the 8HP75 function properly.You can make the 8HP70 and 8HP90 work though
 

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OP, is your pickup a 2x or 4x4?
 
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Steve White

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So I checked the tailshaft of the 66 to the 65 and that is the biggest difference in them. The tailshaft on the 66 is alot bigger then the 65. That is going to be the biggest problem i run into. I currently have contacted advanced adapters to see if they can help so you dont have to change t-cases and also the driveshafts. If i cant get this conversion to work I will have a freshly rebuilt 66RFE that I will be selling. But i will keep everyone updated on my progress.
 

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Would a revised transfer case bolt in (that would fit a 66RFE)? Seems a transfer case out of a 3/4 ton would somewhat easy to get (used). Driveshafts can be dealt with. But like you say, then the project suddenly got bigger.

BTW, besides it being a more robust transmission, what (if any) improved driveability would a guy gain going from a 65 to a 66? I see the gear ratios are different, but.. do they shift better, or tighter/smoother? I'm just curious (not being a jerk)..
 
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Steve White

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So after the weekend I tried to contact Advanced Adapters in California and they said that at this time they are no doing anything to help with this conversion. So I have done some more research and found that the 66RFE output shaft is a 29 spline and is 1.4670" in diameter. The 65 RFE is a 23 spline and is smaller. I do not have the measurement for the 65 diameter as I am finding that unless you use the t-case from a 66 and then change driveshafts as well. The problem you run into with that is the size difference between the two t-cases is pretty substantial. The physical size is also bigger and dont think that it would work in a 1500 frame. If someone knows of a machine shop that would take the input gear for the BW 44-45 t-case and machine it to work with the 66 tailshaft then it would work. or if someone could take the output shaft for the 66 and take it down to the 65 rfe shaft size and splines. That is the only way that i see this conversion could work. I know alot of people are talking about doing the 8 speed conversion but I am not going to do that as then you have to change the cooler lines, cooler, and transmission wiring. So I do not believe that this conversion is going to be possible at this time until someone has the money or a machine shop that would be willing to take on this challenge. Hope this helps as I was really hoping to try it but I dont want to change t-cases and driveshafts. If it was a 2WD then the conversion would be pretty straight forward as you dont have to mess with much and only change the rear drive shaft yoke. Then it is possible. BUT for us with 4WD it is currently not possible.
 
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Steve White

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Would a revised transfer case bolt in (that would fit a 66RFE)? Seems a transfer case out of a 3/4 ton would somewhat easy to get (used). Driveshafts can be dealt with. But like you say, then the project suddenly got bigger.

BTW, besides it being a more robust transmission, what (if any) improved driveability would a guy gain going from a 65 to a 66? I see the gear ratios are different, but.. do they shift better, or tighter/smoother? I'm just curious (not being a jerk)..
I dont think that there would be any driveability differences going from the 65 to the 66. I would think the biggest thing you would notice is reverse of course and then maybe a little better fuel mileage with the deeper 6th gear.
 

Wild one

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So after the weekend I tried to contact Advanced Adapters in California and they said that at this time they are no doing anything to help with this conversion. So I have done some more research and found that the 66RFE output shaft is a 29 spline and is 1.4670" in diameter. The 65 RFE is a 23 spline and is smaller. I do not have the measurement for the 65 diameter as I am finding that unless you use the t-case from a 66 and then change driveshafts as well. The problem you run into with that is the size difference between the two t-cases is pretty substantial. The physical size is also bigger and dont think that it would work in a 1500 frame. If someone knows of a machine shop that would take the input gear for the BW 44-45 t-case and machine it to work with the 66 tailshaft then it would work. or if someone could take the output shaft for the 66 and take it down to the 65 rfe shaft size and splines. That is the only way that i see this conversion could work. I know alot of people are talking about doing the 8 speed conversion but I am not going to do that as then you have to change the cooler lines, cooler, and transmission wiring. So I do not believe that this conversion is going to be possible at this time until someone has the money or a machine shop that would be willing to take on this challenge. Hope this helps as I was really hoping to try it but I dont want to change t-cases and driveshafts. If it was a 2WD then the conversion would be pretty straight forward as you dont have to mess with much and only change the rear drive shaft yoke. Then it is possible. BUT for us with 4WD it is currently not possible.
 

mtofell

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The main reason I thought about doing it cause I have a free 66RFE that i only need to put about $300 bucks into.
I suppose I can see if it doesn't cost you much but beware, the 66rfe is kind of crappy tranny. As they age in these trucks there are starting be a lot of reports of valve body failures. Also, the ratios suck if you will be towing heavy. The ratios are borrowed from from the 68rfe that goes in the Cummins and are really meant for a diesel with the low end torque.

The biggest problem is the gap between 1 and 2. With the 3.73 rear end in my truck and hauling my 5th wheel (11,000#) up a winding grade I'm either at 5000RPMs in 1st or if I let it drop into second it can't hold it and slams back to 1st. Basically, there's a "no man's land" between about 30 and 45 up a hill pulling heavy. Once over 45 where it can hold second it's a beast but it just kind sucks at lower speeds. Granted, and 11K 5th wheel is pretty much pushing the limits of the truck but all reports are the 8-spd is another world entirely.
 

Hemi395

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Would a revised transfer case bolt in (that would fit a 66RFE)? Seems a transfer case out of a 3/4 ton would somewhat easy to get (used). Driveshafts can be dealt with. But like you say, then the project suddenly got bigger.

BTW, besides it being a more robust transmission, what (if any) improved driveability would a guy gain going from a 65 to a 66? I see the gear ratios are different, but.. do they shift better, or tighter/smoother? I'm just curious (not being a jerk)..
The main difference in drivability between the two is the 66 shifts through all 6 gears in sequence. The 65 shifts 1-2-4-5-6 and only goes into 3rd during certain kickdown situations. You can tune the 65 to shift in sequence but the 2-3 shift isn't spaced as nice as the 66 is.

Other than that I doubt you would notice too much difference besides the much shorter reverse ratio.
 
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