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Anyone convert a 3.21 to 3.92?

ksn240

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Okay. I have a 3.92 rear end and I have 8 gears. If I had gotten a 3.21 rear end, how many gears would I have?
8. The only thing that changes is the size of the gears, not the number of them.
 

NDanecker

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Okay. I have a 3.92 rear end and I have 8 gears. If I had gotten a 3.21 rear end, how many gears would I have?

Ok..let me rephrase. You have more gears to play with driving the same speed. For example. with the 3.21 you will likely be stuck in 5 or 6 while cruising on the highway. At same speed with 3.92 you will be in 6 to 7. Gears are closer together and you have 'more gears'. Why do you think Ford went with the 10 speed????

Obviously its still just a 8 speed, but if you never get to 7 (or 8) because the motor needs to run 3k+ rpms to keep the load moving then gear changes will be easier (and you don't have to spin up the motor to compensate on the torque loss from reduced RPMs created by a lower gear ratio).
 

silver billet

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Ok..let me rephrase. You have more gears to play with driving the same speed. For example. with the 3.21 you will likely be stuck in 5 or 6 while cruising on the highway. At same speed with 3.92 you will be in 6 to 7. Gears are closer together and you have 'more gears'. Why do you think Ford went with the 10 speed????

Obviously its still just a 8 speed, but if you never get to 7 (or 8) because the motor needs to run 3k+ rpms to keep the load moving then gear changes will be easier (and you don't have to spin up the motor to compensate on the torque loss from reduced RPMs created by a lower gear ratio).

What matters is the final gear ratios. You're not going to notice a meaningful difference between them at city or highway speed, just that initial tug from 1-2-(3). The remaining 5-6 hears are so close it hurts, just offset by 1 gear (so 3.21 in 7 = 3.92 in 8, 3.21 in 6 = 3.92 in 7, etc).
 

Sonnie

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I don't like the 3.92 in my 2020... don't tow and don't need it. I liked the 3.21 that was in my 2019.

Can we swap everything out? lol
 

StuartV

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I don't like the 3.92 in my 2020... don't tow and don't need it. I liked the 3.21 that was in my 2019.

Can we swap everything out? lol

Yes. As long as by "everything" you mean everything between the front and rear bumpers, inclusive... LOL ;)

I feel the same as you. I have 3.92 gearing and that is the only mechanical thing I would have preferred to be different when I bought my truck. I had an '09 with 3.92 that I traded in. I really thought between 10 years of refinement of the engine, going from a 6 speed to an 8 speed transmission, and adding e-Torque, I would see an actual, meaningful improvement in mileage. That is how I convinced myself to get another 3.92, instead of holding out for the right truck with a 3.21. Instead, I get exactly the same as what I got in my old truck (as far as I can tell).

I do love my truck. But, for what I spend at the pump, I feel like I should have at least 50 more HP to play with.
 

ksn240

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Yes. As long as by "everything" you mean everything between the front and rear bumpers, inclusive... LOL ;)

I feel the same as you. I have 3.92 gearing and that is the only mechanical thing I would have preferred to be different when I bought my truck. I had an '09 with 3.92 that I traded in. I really thought between 10 years of refinement of the engine, going from a 6 speed to an 8 speed transmission, and adding e-Torque, I would see an actual, meaningful improvement in mileage. That is how I convinced myself to get another 3.92, instead of holding out for the right truck with a 3.21. Instead, I get exactly the same as what I got in my old truck (as far as I can tell).

I do love my truck. But, for what I spend at the pump, I feel like I should have at least 50 more HP to play with.
I’ve got the 3.21s. I’d be willing to swap anybody for 3.92s
 

Sonnie

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I would definitely do it if it could be done. I assume if it could be, only labor would be charged since all the parts are there.
 

Scram1500

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I have a simple solution... get smaller tires lol. If you're on an OEM 32 inch tire, going down to a 26 inch tire will convert your 3.21 to a 3.92
 

Sonnie

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So I can go up from 32 to 38 and that will convert mine from a 3:92 to a 3:21... neat... gotta try it.

I've already gone up to a 33, so I am at 3:80 right now... this is so cool.
 

Scram1500

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So I can go up from 32 to 38 and that will convert mine from a 3:92 to a 3:21... neat... gotta try it.

I've already gone up to a 33, so I am at 3:80 right now... this is so cool.
If you're looking to gain mpg the problem is increased weight. 38 inch tires will likely be twice as heavy as a 32 or 33, at over 80 lbs per tire. Mpg's will approach single digit even with the higher ratio. You have the right truck to do it though, it would be awful to have 38's with a 3.21

 

kraftwerx

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I looked into this and talked to a local shop about it, when I found a truck with everything I wanted on it minus the 3.92 for towing. It turns out because of the welded front gear box, FCA will only sell a new front end assembly. I pushed the dealer to make this happen if they wanted my business but we were talking ~3500, so they ended up walking away when I wouldn't eat that cost for a 500 option when doing a factory order.

Hope this helps.
 

Sonnie

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Yeah... I'll live with it for now. I am averaging between 20-21 mpg fairly regularly, so it's not too bad. I suppose if I ever do need to tow or get into the gas, I'll have the right gear to do it.
 

brian42

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Neurobit

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This should help...

 

silver billet

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I would not even think about swapping. The 3.21 is amazing already "as is". Let's compare the 3.21 vs the Toyota 6 Speed, with 2 different gear ratios, since so many people seem to think the Tundra is some powerhouse tow machine:

Screenshot_2020-07-02_20-51-37.png

We (2019 ram's) have the 8HP75 transmisssion. This means that with 3.21s, we have a far higher first gear ratio 16.05 vs the Tundra with even the 4.30 rear end. We have more gears/spread to play with in the midrange (while towing in city/highway), and we have a lower final gear ratio for unloaded highway runs.

Yes; our 3.21 with the 5.7 will feel more powerful, and get better MPG than anything from Toyota.

GM/Ford with the 10 speed? Haven't looked, might do that next for giggles. But I suspect most people that "regret" the 3.21 do it because they think the 3.92 is also there as an option and they should have got that because it's higher ratio yadda yadda. But do the math, and it shows it's a very competitive performer.

I just came home from a 3 hour trip, got down to 9.4 L/100KM on the highway (25 mpg). I would love this thing to death if I could get rid of my ticking issue.
 

Ellisstrong

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I would definitely do it if it could be done. I assume if it could be, only labor would be charged since all the parts are there.
We need to set this swap up. I’m told I need to go to a higher gear when I put my big rims back on. Lol
 

Sonnie

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I would do it in a heartbeat if I knew we could make it happen... and you're not that far away.
 

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