Rear Differential Swap From 3.21 to 3.92

[R1200GS]

I have an '18 1500 Limited 5.7L Hemi 4x4 Crew Cab with the 3.21 rear end ratio, along with the limited slip differential. The 3.21 rear end ratio is the only thing that I do not care for about this truck. I've had the 3.92 in other 1500 Rams and I prefer its ratio for a number of reasons. Does anyone have experience in having the rear ratio changed from the 3.21 to the 3.92 and if so, what issues arise that must be addressed such as speedometer/tachometer recalibration, etc. Thank you!

 

[Lee Peterson]

If you do the rear you have to do the front. The front requires a whole new differential I think as ring gear is welded on. Speedometer will be fine gear change don't effect it.

 

[Wild one]

I have an '18 1500 Limited 5.7L Hemi 4x4 Crew Cab with the 3.21 rear end ratio, along with the limited slip differential. The 3.21 rear end ratio is the only thing that I do not care for about this truck. I've had the 3.92 in other 1500 Rams and I prefer its ratio for a number of reasons. Does anyone have experience in having the rear ratio changed from the 3.21 to the 3.92 and if so, what issues arise that must be addressed such as speedometer/tachometer recalibration, etc. Thank you!

You're best option,might be to search the wrecking yards for a low milege wrecked truck with 3.92's,and buy both front and rear diffs,then swap them out,and sell your 3.21 diffs.As stated above,you have to buy the complete diff for the front,as you can't swap gears in the front ZF diffs,you have to swap the complete front pumpkin.Good news is you can re-use your front driveshaft / axles and actuator if needed.

 

[CheechDogg.0n37s]

I have an '18 1500 Limited 5.7L Hemi 4x4 Crew Cab with the 3.21 rear end ratio, along with the limited slip differential. The 3.21 rear end ratio is the only thing that I do not care for about this truck. I've had the 3.92 in other 1500 Rams and I prefer its ratio for a number of reasons. Does anyone have experience in having the rear ratio changed from the 3.21 to the 3.92 and if so, what issues arise that must be addressed such as speedometer/tachometer recalibration, etc. Thank you!

You will have to adjust the speedometer, I used alfaobd and you will need a security bypass harness to do so. You will also have to set the gears from 3.21s to 3.92s something that you can also do with alfaobd.

I regeared from 3.21s to 4.88s and I have had no issues, I just fixed both speedometer and gear ratio settings with alfaobd, took maybe 10 minutes to do so and it's been good ever since ...

 

[Wild one]

You will have to adjust the speedometer, I used alfaobd and you will need a security bypass harness to do so. You will also have to set the gears from 3.21s to 3.92s something that you can also do with alfaobd.

I regeared from 3.21s to 4.88s and I have had no issues, I just fixed both speedometer and gear ratio settings with alfaobd, took maybe 10 minutes to do so and it's been good ever since ...

As long as he doesn't change tire size,his speedo will be okay,the only thing that fubars the speedo is if you swap on bigger or smaller diameter tires,but to keep the tranny happy,he should change the gear ratio via AlphaOBD as you stated.Good point,CheechDogg

 

[CheechDogg.0n37s]

As long as he doesn't change tire size,his speedo will be okay,the only thing that fubars the speedo is if you swap on bigger or smaller diameter tires,but to keep the tranny happy,he should change the gear ratio via AlphaOBD as you stated.Good point,CheechDogg

Correct, I didn't mention the change in tire size ....

 

[scott lass 18]

I have an '18 1500 Limited 5.7L Hemi 4x4 Crew Cab with the 3.21 rear end ratio, along with the limited slip differential. The 3.21 rear end ratio is the only thing that I do not care for about this truck. I've had the 3.92 in other 1500 Rams and I prefer its ratio for a number of reasons. Does anyone have experience in having the rear ratio changed from the 3.21 to the 3.92 and if so, what issues arise that must be addressed such as speedometer/tachometer recalibration, etc. Thank you!

Did my change out 10 years ago love the 3.92. Soooo much better. Got 17 to 21 mpg on freeway. Loaded up . Best thing I ever did for my truck !

 

[Ghost1500]

How do you know if the truck comes with 3.92's?

Best regards,

-Mike

 

[CheechDogg.0n37s]

How do you know if the truck comes with 3.92's?

Best regards,

-Mike

Go to this link and at the end after the = input your VIN number

https://www.jeep.com/webselfservice/BuildSheetServlet?vin=

 

[Ghost1500]

Link doesn't work for me.

-Mike

 

[CheechDogg.0n37s]

Link doesn't work for me.

-Mike

Go to it and add your vin to the end of the link ....

 

[Ghost1500]

Thank you! Mine has the 3.21's. Oh well....

Best regards,


-Mike

 

[CheechDogg.0n37s]

Thank you! Mine has the 3.21's. Oh well....

Best regards,


-Mike

Well time to re-gear ! lol ...

What size wheels you got now? if you don't have 35s no reason to re-gears ..

When my truck was stock it has 3.21s and I love it, lifted the beast and added the 20x12s with 37s... I did drive it like that for a few months and it wasn't that bad, but overpasses, uphills with curves, no way! it was a horrible lol ..

I got to drive a few trucks that had 4.56s, 4.10s and a 2500 with the 4.88s ... dude that re-gear said to go with the 4.88s because of the 37s, I was going to just go 4.56s but felt how the 2500 rocked with the 4.88s and I knew that the 1500 with 4.88s with 37s was going to be just right lol ..

But yes, those 3.21s were a lot of fun when my truck was stock and I got the same mpg with her then as I do now with the 4.88s, 13-15 mpg in town and up to 20 on long highway only trips ..... and a lot has to do with the fact that it's so fun driving it with the 4.88s, lifted and with a Carven Competitor series exhaust lol !!!!

 

[aj_g]

How do you know if the truck comes with 3.92's?

Best regards,

-Mike

You can just put in the VIN on this website after selecting "Dodge" in the list

Window Sticker Lookup by VIN

Use your VIN number to get your Chrysler, Dodge, Ford, or Jeep Window Sticker

windowstickerlookup.com

 

[GTyankee]

follow the instructions & Use CAPITAL LETTERS

FCA RAM

fcacommunity.force.com

 

[Ghost1500]

Any idea how much a diff gear swap goes for? I know I have to do front and back. Thank you in advance.

-Mike

 

[Wild one]

Any idea how much a diff gear swap goes for? I know I have to do front and back. Thank you in advance.

-Mike

Cheapest way is to buy both diffs from a wrecking yard,swap them on your driveway,then sell your 3.21 diff's. You can't swap gears in your front diff,you have to buy the complete front pumpkin to change the front,so you might as well buy a complete rear diff also.You'd get more bang for your buck by swapping the stock torque convertor for a 3,000 to 3500 rpm stall convertor,and then get a good tune for the engine and transmission. The 8 speeds convertor is not like your old style convertor,it can be tuned slightly as far as when and what gears you want it to lock up in

 

[Meatman8769]

Well time to re-gear ! lol ...

What size wheels you got now? if you don't have 35s no reason to re-gears ..

When my truck was stock it has 3.21s and I love it, lifted the beast and added the 20x12s with 37s... I did drive it like that for a few months and it wasn't that bad, but overpasses, uphills with curves, no way! it was a horrible lol ..

I got to drive a few trucks that had 4.56s, 4.10s and a 2500 with the 4.88s ... dude that re-gear said to go with the 4.88s because of the 37s, I was going to just go 4.56s but felt how the 2500 rocked with the 4.88s and I knew that the 1500 with 4.88s with 37s was going to be just right lol ..

But yes, those 3.21s were a lot of fun when my truck was stock and I got the same mpg with her then as I do now with the 4.88s, 13-15 mpg in town and up to 20 on long highway only trips ..... and a lot has to do with the fact that it's so fun driving it with the 4.88s, lifted and with a Carven Competitor series exhaust lol !!!!

What do all these numbers in gear ratios mean to a rookie just trying to learn about his truck. I know mine has the 3.92 ratio, but it's not something I looked for when buying my truck, actually just found out with the link you provided. I guess what I'm asking is what does this do ?

 

[ramffml]

Conceptually... both 3.21 and 3.92 trucks give you 8 gears to play with, but the 3.92 has gears:
a, b, c, d, e, f, g, h
the 3.21 has:
b, c, d, e, f, g, h, i

so they "share" 6 middle gears "b - h", the 3.92 has an extra lower gear, "a", but misses the top gear from the 3.21, "i". (yes I'm aware that mechanically this is not 100% accurate, but its close enough for somebody trying to figure out which gears to get for their truck)

What it means:
the 3.92 will pull a heavier load quicker from a dead stop
the 3.21 will get better MPG on the freeway or anytime it is in 8th gear.

Pros and cons to both, this forum seems to hate the 3.21 but it's a great option for the vast majority of people out there who only use their truck for commuting, or for towing 0 to 7000 pounds. The 3.92 is better for offroading (rebel!), guys who run bigger tires, or guys who reallly want that last bit of power when accelerating from a dead stop.

 

[CheechDogg.0n37s]

What do all these numbers in gear ratios mean to a rookie just trying to learn about his truck. I know mine has the 3.92 ratio, but it's not something I looked for when buying my truck, actually just found out with the link you provided. I guess what I'm asking is what does this do ?

If you don't have a lifted truck with 35s or you tow anything either 3.21s or 3.92s are fine ...

If you go up to 35" tires the recommended gears are 4.56s

If you go up to 37" tires the recommenced gears are 4.88s but you can only find certain shops that will do it on any Ram 1500, so 4.56s are fine with the 37s ..

But you can still get away with just 3.21s or 3.92s with 35s if you don't tow or you live in a pretty flat area ....