An Engineer's Ultimate Guide To 3.21 VS 3.92 Axle Ratio

[BowDown]

The 3.92 is faster off the line and most likely to 60 and 100MPH but I haven't seen a drag race comparison. With the eTorque and 3.92 you have a even slightly quicker off the line also.

I think the 3:92 truck would be faster to 30mph but beyond that, the 8 speed trans helps keeps the engine in its torque peak more than gears do and the advantage diminishes.

 

[User_3336]

I test drove the SAME truck, one with a 3.21 and one with the 3.92. I would NOT buy a truck with a 3.21 axle ratio in a RAM.

Going up several inclines and minor hills, the 3.92 stayed in 8th at 60mph.
Same path back, 3.21 continually back and forth between 7th and 8th, and went from 8th to 5th when experiencing any minor change in payment like a small incline or hill. this would literally drive me crazy all the time.

3.92 hovers around 2000rpm at 60mph (or little less).

the $95 option was well worth it, even for a daily driver.

 

[silver billet]

I test drove the SAME truck, one with a 3.21 and one with the 3.92. I would NOT buy a truck with a 3.21 axle ratio in a RAM.

Going up several inclines and minor hills, the 3.92 stayed in 8th at 60mph.
Same path back, 3.21 continually back and forth between 7th and 8th, and went from 8th to 5th when experiencing any minor change in payment like a small incline or hill. this would literally drive me crazy all the time.

3.92 hovers around 2000rpm at 60mph (or little less).

the $95 option was well worth it, even for a daily driver.

This comes up time and again. Put the 3.21 in 7th and it's exactly the same final gear ratio as the 3.92 in 8th. So just lock it out of 8th if it bothers you that much.

6th in the 3.21 is literally identical to 7th in the 3.92.

Once it sinks in that the top 2/3's of the gears are either very close or exactly the same as the gears in the 3.92, just 1 gear behind, you will realize that the downshifting behaviour you attribute to being a 3.21 problem would ocurr in the 3.92 as well. This is called "confirmation bias". You expect the 3.21 to be far worse, so the second it downshifts (not including 8th) then you blame it on what you expect it to be (gear ratio) rather than what is is (not enough torque; the 3.92 would have to downshift too).

The 3.92 just gets you out of the hole quicker. That's it. Once the trucks are in third or beyond, they are pretty much a direct match for eachother until the 3.92 runs out of gears and the 3.21 has one upshift left. They're just offset by 1 gear.

I mean look how close those numbers are below. You're trying to tell me you notice a difference of 0.12 in final ratio?? Not likely. Even more so for 0.0044.

 

[Willwork4truck]

I test drove the SAME truck, one with a 3.21 and one with the 3.92. I would NOT buy a truck with a 3.21 axle ratio in a RAM.

Going up several inclines and minor hills, the 3.92 stayed in 8th at 60mph.
Same path back, 3.21 continually back and forth between 7th and 8th, and went from 8th to 5th when experiencing any minor change in payment like a small incline or hill. this would literally drive me crazy all the time.

3.92 hovers around 2000rpm at 60mph (or little less).

the $95 option was well worth it, even for a daily driver.

The v6 did to me what you described. The 3.21 hemi we have only drops a gear on an incline or under moderate to hard accel, as I’d expect a tall-geared truck to do.

I don’t find it very irritating and it allows for the better mpg’s, but then I don’t tow anymore, haul heavy, or live in a hilly area.

It still was a difficult choice for me to make (gearing) as I’d happily have taken a 3.55 (4th gen) but had trouble accepting the mpg’s of the 3.92 given our style of driving/use. Even a year ago April the gearing mpg controversy was raging.

The final thing for us was the 3.92 truck deal fell through and the one she liked next was a 3.21 so... As you can see I had accepted a 3.92 but fate intervened.

 

[Boston]

Bet 99% of buyers have no clue what 3.21 and 3.92 mean. Their eyes probably glaze over and skip that part of the window sticker.
I’ve only driven the 3.21 and that was great.

 

[User_3336]

They will learn what "gear ratio" means when the transmission can't determine what gear to stay in.
that 7-8 continual hunt, and the 8-5 downshift in the 3.21 is awfully annoying.

Bet 99% of buyers have no clue what 3.21 and 3.92 mean. Their eyes probably glaze over and skip that part of the window sticker.
I’ve only driven the 3.21 and that was great.

 

[BigD]

I've watched a lot of You Tube videos on 3.21 vs. 3.92 they say the mpg difference is between 1 & 1.5 mpg. That ain't much. I'm averaging 19 mpg with my truck. I drive 50/50 city/highway

 

[User_3336]

i'm just really impressed how the 3.92 on the Crew Hemi holds 8th on 90% of my driving. I am not concerned about the extra RPM or gas.

I've watched a lot of You Tube videos on 3.21 vs. 3.92 they say the mpg difference is between 1 & 1.5 mpg. That ain't much. I'm averaging 19 mpg with my truck. I drive 50/50 city/highway

 

[bluecrewnuke]

I traded a few notes with the folks over at PML (aluminum diff covers). I shot them a picture of mine, and they think my axle was made by Dodge.

 

[BowDown]

I've watched a lot of You Tube videos on 3.21 vs. 3.92 they say the mpg difference is between 1 & 1.5 mpg. That ain't much. I'm averaging 19 mpg with my truck. I drive 50/50 city/highway

Its more than that, there's a 22% gear rate split between the 2 gear sets, that's closer to 3-4 mpg or about 500 rpm. Anyone that thinks 500 rpm doesn't make a difference, drive at 1500 rpm and observe your instant mileage the drive at 1900 -2000 rpm.

 

[BigD]

Its more than that, there's a 22% gear rate split between the 2 gear sets, that's closer to 3-4 mpg or about 500 rpm. Anyone that thinks 500 rpm doesn't make a difference, drive at 1500 rpm and observe your instant mileage the drive at 1900 -2000 rpm.

I set my trip b4 work this morning. 65 mile round trip driving 75 mph half the time, & then driving 60 half the time. I got 19.6 mpg !!

 

[BowDown]

I set my trip b4 work this morning. 65 mile round trip driving 75 mph half the time, & then driving 60 half the time. I got 19.6 mpg !!

I had a 3:21 geared laramie while my wife's car was in the shop, in the city, the truck averaged 17-18 on the highway it was 21-22. Non etorque HEMI. I have a 3:92 geared Ram now and the 3:21 truck felt stronger but this is not nor will ever be a performance truck. I can get in my car for that anyway

 

[Boston]

As someone who is still waiting to pull the trigger, this is a fun conversation to watch in which everyone argues they are right.

Someone else mentioned the 3.55 ratio. I don't know the MPH/1000 revs in any gear regardless of rear end ratio. But have to think that 3.55 might well be a great balance between the two ratios. Ram can include the 3.92 as part of the towing package along with the TBC and elephant ears.
But that might just create an additional argument.....

 

[SpeedyV]

As someone who is still waiting to pull the trigger, this is a fun conversation to watch in which everyone argues they are right.

Someone else mentioned the 3.55 ratio. I don't know the MPH/1000 revs in any gear regardless of rear end ratio. But have to think that 3.55 might well be a great balance between the two ratios. Ram can include the 3.92 as part of the towing package along with the TBC and elephant ears.
But that might just create an additional argument.....

That’s right - it's an idea that's been proposed on many threads...but since it's not available with a Hemi, the point is moot.

 

[392DCGC]

They will learn what "gear ratio" means when the transmission can't determine what gear to stay in.
that 7-8 continual hunt, and the 8-5 downshift in the 3.21 is awfully annoying.

No, it's really not annoying and you're exaggerating. My 3.21 does not gear hunt constantly, and between the stereo and eTorque, I don't even notice shifts. 3.92 holds 8th gear longer than a 3.21? Cool story man. The 99% of the time my 3.21 is in 8th gear, I'm saving more fuel and running the engine at lower RPM.

 

[User_3336]

your experience in a vehicle with ETORQUE might be different.

I would not have said it was ANNOYING if it wasn't, and I'm surely not exaggerating. Its the sole reason I went with a 3.92 axle instead of the 3.21 after driving BOTH back to back on the same round trip.

No, it's really not annoying and you're exaggerating. My 3.21 does not gear hunt constantly, and between the stereo and eTorque, I don't even notice shifts. 3.92 holds 8th gear longer than a 3.21? Cool story man. The 99% of the time my 3.21 is in 8th gear, I'm saving more fuel and running the engine at lower RPM.

 

[silver billet]

your experience in a vehicle with ETORQUE might be different.

I would not have said it was ANNOYING if it wasn't, and I'm surely not exaggerating. Its the sole reason I went with a 3.92 axle instead of the 3.21 after driving BOTH back to back on the same round trip.

You're welcome to your opinion, but the math clearly shows you're wrong. Look at the chart I posted above.

Having owned my truck (3.21) for a year, and done pretty much all the different types of driving out there in it, anything making my truck downshift from 8 to 5 is going to make your truck downshift heavily too. That is a severe increase in required power, your 3.92 is going to be sweating just as bad, sorry. Most likely you'd be in 6th.

 

[Boston]

Was looking at towing capacity and Ram specs the 3.92 equipped trucks way higher than 3.21. Typically 10k plus versus under 9 on the trucks I was looking at.

Can't see my towing exceeding 6500lbs anyway so I'm good with the lower numbers from 3.21

 

[Jag1623]

Excellent post!

 

[Jack]

This comes up time and again. Put the 3.21 in 7th and it's exactly the same final gear ratio as the 3.92 in 8th. So just lock it out of 8th if it bothers you that much.

6th in the 3.21 is literally identical to 7th in the 3.92.

Once it sinks in that the top 2/3's of the gears are either very close or exactly the same as the gears in the 3.92, just 1 gear behind, you will realize that the downshifting behaviour you attribute to being a 3.21 problem would ocurr in the 3.92 as well. This is called "confirmation bias". You expect the 3.21 to be far worse, so the second it downshifts (not including 8th) then you blame it on what you expect it to be (gear ratio) rather than what is is (not enough torque; the 3.92 would have to downshift too).

The 3.92 just gets you out of the hole quicker. That's it. Once the trucks are in third or beyond, they are pretty much a direct match for eachother until the 3.92 runs out of gears and the 3.21 has one upshift left. They're just offset by 1 gear.

I mean look how close those numbers are below. You're trying to tell me you notice a difference of 0.12 in final ratio?? Not likely. Even more so for 0.0044.

View attachment 57024

I think I calculated the gear ratio based off the older 8HP70 trans. Good to know they have a wider gear ratios in the newer one.