Rear Differential Swap From 3.21 to 3.92

[GTyankee]

simple way to think of gear ratios

3:21 will give you the very best mileage when empty, but it will only pull a Harbor Freight sized trailer

3:55 will not give you quite as much MPG as above, but it will pull a larger trailer

3:92 loses a bit more MPG, but accelerates real quick & will easily pull 7,000, while climbing a grade

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Going with a larger tire circumference also changes the gear ratio, but you would need a mathematical formula to figure it out.

Lifting your vehicle cost you MPG

 

[Wild one]

simple way to think or gear ratios

3:21 will give you the very best mileage when empty, but it will only pull a Harbor Freight sized trailer

3:55 will not give you quite as much MPG as above, but it will pull a larger trailer

3:92 loses a bit more MPG, but accelerates real quick & will easily pull 7,000, while climbing a grade

=================================
Going with a larger tire circumference also changes the gear ratio, but you would need a mathematical formula to figure it out.

Lifting your vehicle cost you MPG

If you're pulling 7,000lbs in 7th with 3.92's,please explain the differance between pulling 7,000lbs in 6th with 3.21's to me,when both are virtually identical final drive ratio's. To many guys get hung up on diff gear ratio's,when it's the overall final drive ratio,they should be worry'ing about.To get final drive ratio's,you mutliply the transmission gear ratio by the rear-end ratio

3.92's are roughly 17.5% deeper then 3.21's.
6th gear is roughly 16% lower then 7th gear
1.5% is not enough of a differance to even notice
Once you shift into 2nd with 3.92's the 3.21's that are still in 1st,have the torque advantage ,only place the 3.92's shine is getting that 7,000lbs moving,after it's moving they start to lose their advantage

TorqueFlite 8 (8HP70) Ratios​

1st​	2nd​	3rd​	4th​	5th​	6th​	7th​	8th​	R​
4.71 : 1​	3.14 : 1​	2.10 : 1​	1.67 : 1​	1.29 : 1​	1.00 : 1​	0.84 : 1​	0.67 : 1​	3.30 : 1​

 

[Meatman8769]

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 ....

Thank you for spelling it out for me!

 

[Meatman8769]

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.

Appreciate the explanation! Thanks

 

[Meatman8769]

All makes sense to me. I do pull my work trailer a few times a month, only have my truck levelled and run 33s. Always felt like my truck drives top knotch. Definitely accelerates great minus the bit of throttle lag, which will soon be corrected

 

[GTyankee]

doesn't the 3.92 really shine pulling that weight up a grade
the steeper the grade, the more it shines

California State & Federal Highways hold the grades to 6%

County roads can be even steeper, one in California has a 26 degree grade

Highway grades are pretty easy to understand and expressed in 100-unit increments. For example, a highway with a six-percent grade means it gains (or loses) no more than six feet of elevation for every 100 feet of distance.

8 of the Steepest and Most Extreme Highways in North America for Thrill-Seeking Road Trippers

These 8 roads have the steepest highway grades in the US and are seriously no joke. Here's where they are, how to drive them, and where to stop on the way.

matadornetwork.com

 

[Wild one]

doesn't the 3.92 really shine pulling that weight up a grade
the steeper the grade, the more it shines

California State & Federal Highways hold the grades to 6%

County roads can be even steeper, one in California has a 26 degree grade

Highway grades are pretty easy to understand and expressed in 100-unit increments. For example, a highway with a six-percent grade means it gains (or loses) no more than six feet of elevation for every 100 feet of distance.

8 of the Steepest and Most Extreme Highways in North America for Thrill-Seeking Road Trippers

These 8 roads have the steepest highway grades in the US and are seriously no joke. Here's where they are, how to drive them, and where to stop on the way.

matadornetwork.com

I know what grade percentages are,but you don't quite comprehend what final drive ratio's are. Once you're past 1st gear with your 3.92's,and you factor in the transmission ratio's,the 3.21's start to look better.You're assuming you're pulling a 6% grade in 8th,and there's no 8 speed 1500 pulling 7,000lbs up a 6% grade in 8th.
Lets say you're managing to pull that load in 5th with 3.92's which equals roughly a 5.06:1 final drive ratio,while a 3.21 in 4th equals a 5.36:1 final drive ratio.The 3.21's actually have a slight torque advantage.
It's only when you're trying to get that 7,000lbs moving that the 3.92's hold the advantage,once rolling and past 1st gear the 3.92's advantage becomes a mute point.

 

[ramffml]

If you're pulling 7,000lbs in 7th with 3.92's,please explain the differance between pulling 7,000lbs in 6th with 3.21's to me,when both are virtually identical final drive ratio's. To many guys get hung up on diff gear ratio's,when it's the overall final drive ratio,they should be worry'ing about.To get final drive ratio's,you mutliply the transmission gear ratio by the rear-end ratio

3.92's are roughly 17.5% deeper then 3.21's.
6th gear is roughly 16% lower then 7th gear
1.5% is not enough of a differance to even notice
Once you shift into 2nd with 3.92's the 3.21's that are still in 1st,have the torque advantage ,only place the 3.92's shine is getting that 7,000lbs moving,after it's moving they start to lose their advantage

TorqueFlite 8 (8HP70) Ratios​

1st​	2nd​	3rd​	4th​	5th​	6th​	7th​	8th​	R​
4.71 : 1​	3.14 : 1​	2.10 : 1​	1.67 : 1​	1.29 : 1​	1.00 : 1​	0.84 : 1​	0.67 : 1​	3.30 : 1​

I've been trying to say this for years now too but some of the 3.92 guys don't want to read it. Probably because I word it so politely and delicately so as to not offend anyone? <--- wee little joke.

Another great way to look at it: "everyone knows" the old v8 tundra tows great, right? It has that reputation anyway. The 3.21/8 speed has a deeper first gear, and a taller final gear than the 6 speed tundra with a 4.30 rear end. Yep, it manages to pull harder off the line, while still having a taller OD for fuel efficency.

A third great way to look at it: count up the time you spend towing in first gear, vs driving in 8th gear. My truck lives in 8th gear, and spends minutes a year towing in first gear. Why worry so much about that initial tug off the line while towing, when 95+% of the time my truck is cruising empty in 8th?

 

[Docwagon1776]

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 ?

It's the ratio of how many times the drive shaft rotates vs how many times the tire rotates and therefore works as a torque multiplier.

 

[Meatman8769]

It's the ratio of how many times the drive shaft rotates vs how many times the tire rotates and therefore works as a torque multiplier.

Haha someone finally says it in English! I did understand before, but this really really simplifies it

 

[Ghost1500]

Once you shift into 2nd with 3.92's the 3.21's that are still in 1st, have the torque advantage, only place the 3.92's shine is getting that 7,000lbs moving, after it's moving they start to lose their advantage.

I like the way you think! That explanation is making me keep my 3.21's! Thank you!

Best regards,

-Mike