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

[Scram1500]

Wanna win stop-light races? 3.92
Tow big/heavy things regularly? 3.92
Mostly highway at 80 mph without a trailer? 3.21

I'd like to add tire roasting to the 3.92 checklist

I doubt you win many stop light races with the 3.92. I can flash the torque converter and mitigate any advantage the 3.92 has from a dead stop.

Can you explain what flash the torque converter means?

 

[brian42]

In the process of ordering a 2021 Laramie. Currently I have a bighorn with 3.21 gears and heavy E tires. I put a lot of highway miles on the truck and the heaviest load it'll carry is a truck bed full of camping gear. I haven't noticed much of an issue with the 3.21 but there are times it seems to stick in a gear that is should move out of going up steady grades. Would there be any reason for me to switch to a 3.92 with the new truck?

Honestly I think the difference is just about negligible. There's a little wiggle room on each side (grunt down low or overall economy). It will come down to what you are used to.

I'm coming from a 4 speed (auto) with 3.73 gears. I was dead-set on 3.92 gears. The only truck that came close to what I was looking for had 3.21s. Out of curiosity I went and test drove it and was pleasantly surprised at how responsive it was. The 8-speed negated the need for shorter gears for me.

Even in stock form I can still spin a tire and get the TCS light with my 3.21 gears.

If I was planning on a lift with bigger tires, moving up to E-rated tires (at OEM size that's 10+ lbs extra per tire), or towing a lot I would have found a truck with 3.92 gears.

This is, of course, is all opinion and my personal reasons for my choice.

 

[JGRaider]

Easy to see there are some passionate people defending their personal choices on this subject. In actuality, I really wanted the 3.21, but found a truck "I couldn't live without" in stock that had the 3.92. I don't tow very often, but I've really enjoyed it, a whole lot actually. This EcoD runs cirlces around the GMC Sierra SLT 4x4 crew with 3.23 that it replaced. I'm averaging about 7mpg better in both city and highway than the GMC did. Seems like a win for me.

 

[silver billet]

In the process of ordering a 2021 Laramie. Currently I have a bighorn with 3.21 gears and heavy E tires. I put a lot of highway miles on the truck and the heaviest load it'll carry is a truck bed full of camping gear. I haven't noticed much of an issue with the 3.21 but there are times it seems to stick in a gear that is should move out of going up steady grades. Would there be any reason for me to switch to a 3.92 with the new truck?

Your "complaint" there is regarding transmission tuning, ie, it doesn't downshift quick enough. That won't change between 3.21 and 3.92. The transmissions are tuned heavily in favour of MPG, so they're always going to want to sit in the highest gear they possibly can, while making just enough power to keep you there.

The difference in these gears is first/second, and eight. Everything else will be a wash, the 3.92 will be in a higher numerical gear than the 3.21, but have an equivalent gear ratio at the wheels; so both trucks will be pretty much same RPM/power at the same MPH once you're moving in the city/highway, until the 3.92 runs out of gears and the 3.21 can upshift one more time.

 

[silver billet]

Easy to see there are some passionate people defending their personal choices on this subject. In actuality, I really wanted the 3.21, but found a truck "I couldn't live without" in stock that had the 3.92. I don't tow very often, but I've really enjoyed it, a whole lot actually. This EcoD runs cirlces around the GMC Sierra SLT 4x4 crew with 3.23 that it replaced. I'm averaging about 7mpg better in both city and highway than the GMC did. Seems like a win for me.

That's not apples to oranges in anyway, which I'm quite sure you realize? EcoD vs v8 is what you're noticing, not 3.23 vs 3.92. Depending on the age of the GMC, you may have gained an extra 2 gears as well.

 

[z0n3]

Your "complaint" there is regarding transmission tuning, ie, it doesn't downshift quick enough. That won't change between 3.21 and 3.92. The transmissions are tuned heavily in favour of MPG, so they're always going to want to sit in the highest gear they possibly can, while making just enough power to keep you there.

The difference in these gears is first/second, and eight. Everything else will be a wash, the 3.92 will be in a higher numerical gear than the 3.21, but have an equivalent gear ratio at the wheels; so both trucks will be pretty much same RPM/power at the same MPH once you're moving in the city/highway, until the 3.92 runs out of gears and the 3.21 can upshift one more time.

Seems like the easiest answer for me then is to stick with 3.21. I dont plan on heavy towing with this truck and dont need to squeeze every bit of payload out of it.

 

[Buz]

I would rather have the 3.21 in my 2020 Limited for more relaxed RPM's, but the dealer had the exact truck I wanted in 3.92. I didn't think twice and bought it.
I'm coming off a 2018 F150 EB (3.55) which had serious low end torque/pull. Even with 3.92 my heavy Ram feels slow at lower RPM's (which it is) compared to my EB.
I could only imagine how the 3.21's would feel TO ME around town compared to my EB.
Even though it's slower 0-60 than my F150, I absolutely LOVE my Ram and it's smoooooooth power delivery. I grew very tired of the finicky, peaky power of a twin turbo/direct inject motor; They are modern marvels for sure, but they absolutely suck if all you want is a nice smooth plush ride. The 10 speed tranny was a bit much too.
So here's my take on the benefits of both axle ratios on our trucks:
3.21
-Bonus overdrive gear.
-Lower, more relaxed RPM's on highway (big plus)
-Better MPG on highway

3.92
-If you really do tow heavy stuff this is the one you want.
-Better for resale. (people look at towing & big tires/lifts)
-Less gear hunting, especially in 8th..
-Makes the truck feel a bit lighter & sportier off the line & around town.

 

[silver billet]

So here's my take on the benefits of both axle ratios on our trucks:
3.21
-Bonus overdrive gear.
-Lower, more relaxed RPM's on highway (big plus)
-Better MPG on highway

3.92
-If you really do tow heavy stuff this is the one you want.
-Better for resale. (people look at towing & big tires/lifts)
-Less gear hunting, especially in 8th..
-Makes the truck feel a bit lighter & sportier off the line & around town.

You're in a limited; check your payload, I bet you can tow < 8000 pounds due to running out of payload, 3.92 isn't going to correct that because you always have to reduce what you can tow until ALL the limits of your truck are in bounds.

There is no less gear hunting with 3.92, the 3.21 doesn't hunt at all. If it downshifts too much for you then you put it in 7 and it's identical to 3.92 in 8th.

Resale is complete speculation on your part. Ask around, most people are more concerned about MPG in a 1500 series truck then they are about getting to 60 a fraction of a second quicker. And for towing, again, 3.92 is only one of the limits you have to remember to stay within. Payload will almost always decide what you can tow, especially in your truck.

The 3.92 is so completely misunderstood.

 

[BowDown]

I'd like to add tire roasting to the 3.92 checklist


Can you explain what flash the torque converter means?

Its a drag race term/application. You have two options with an auto, hold your left foot on the brake and press the gas pedal with your right foot and bring the engine rpm's up to the point that the rear tires want to break traction, that's called stalling out the convertor, loading up the convertor or convertor stall and allows you to leave at a higher RPM closer to the engine torque curve.

Flashing the convertor is the process of placing the left foot on the brake pedal and holding the vehicle in place until last yellow flashes (green light on the street) while the right foot just rests on the gas pedal but doesn't press it. Upon the light changing, you release the brake pedal at the same time you go WOT on the gas pedal.

Not loading the convertor will cause to to "flash" past the advertised stall speed for example. A 2500 rpm convertor will brake stall to approx 26-2700 rpm but if you use the flash process, you will usually gain an additional 2-300 rpm bringing the engine closer to its torque curve than stalling it out would.
What is the difference? Stalling the convertor out preloads the driveline and there's less lost rotational motion when releasing the brake but it may not get you high enough into the torque curve.
Flashing the convertor will get you closer to your torque curve but has the tendency to cause traction loss due to the extra rpm and in some instances you lose a fraction of a second due to the driveline loading up.

In my experience most vehicles perform better from flashing the convertor but unless you race a lot, the average person doesn't know that and would use brake stall vs flashing the convertor.

 

[JGRaider]

That's not apples to oranges in anyway, which I'm quite sure you realize? EcoD vs v8 is what you're noticing, not 3.23 vs 3.92. Depending on the age of the GMC, you may have gained an extra 2 gears as well.

Yeah, I realize that. My GMC was a 2019 model. The Laramie EcoD still runs circles around the GMC and does it much more efficiently, quietly, smoothly, etc.

 

[Jack]

Weren't we talking about a lifted truck and or tires vs a stock truck and air ride? Also, that link is a forum where they guessed the cd, not fact. Lastly, the air ride on the ram impacts cd as lifted trucks have a larger frontal area, did you not watch? Even if you cut the cd down, its still more power loss than 1/2 hp

No we're talking about stock truck with vs without air suspension.

 

[BowDown]

No we're talking about stock truck with vs without air suspension.

Ok, your comparison still netted less than 1/2 hp difference.

 

[vegaskeith]

No one here as I can see has mentioned the fact that you can get a 3.55. For some reason dealers just order the 3.21 or 3.92. Most dealers that have Eco Diesels in stock have the 3.92.

 

[Scram1500]

No one here as I can see has mentioned the fact that you can get a 3.55. For some reason dealers just order the 3.21 or 3.92. Most dealers that have Eco Diesels in stock have the 3.92.

Most of us here are Hemi owners, the 3.55 is not available for the Hemi

 

[JJRamTX]

As quoted above "Stoplight racing" 0-60 time difference with the eTorque and 3.92. I have hit some mid 6's. Last I saw the eTorque and 3.92 gives best times around 6.3, and 3.92 without eTorque 6.5, and the regular hemi 3.21 around 6.7

 

[Rossum]

Last I saw the eTorque and 3.92 gives best times around 6.3, and 3.92 without eTorque 6.5, and the regular hemi 3.21 around 6.7

Anyone else here old enough to remember when any vehicle that could do 0-60 in under 10 seconds was considered fast?

 

[brian42]

Anyone else here old enough to remember when any vehicle that could do 0-60 in under 10 seconds was considered fast?

That was also when you expected a nice car for anything over $10K and 300Hp was only available in the high end sports cars...

 

[flyfingers]

Having read every post in this topic my question would be which rear end ratio would put less of a strain on drive line components overall? I see the 3.92's seem run approx 21% higher in rpms at hwy speeds than the 3.21's and I wonder if, over say 100k highway miles would that engine (geared with 3.92's) technically have 21% more wear / tear than one geared to 3.21?

 

[JJRamTX]

Having read every post in this topic my question would be which rear end ratio would put less of a strain on drive line components overall? I see the 3.92's seem run approx 21% higher in rpms at hwy speeds than the 3.21's and I wonder if, over say 100k highway miles would that engine (geared with 3.92's) technically have 21% more wear / tear than one geared to 3.21?

The hard part to calculate is how much the 3.21 downshifts up a hill or passing when the 3.92 doesn't. I would guess that would negate at least 5-10% of that wear number.

 

[silver billet]

The hard part to calculate is how much the 3.21 downshifts up a hill or passing when the 3.92 doesn't. I would guess that would negate at least 5-10% of that wear number.

Well how about the extra shift each time in the 3.92? Literally every time you get to city speed you will (more than likely) be 1 gear ahead of the 3.21. So if you are sitting in 6th gear, at that exact same speed the 3.21 might only be in 5th.

This is getting ridiculous. Don't worry about drive train strain, I've never even heard of that being a problem for any truck/gears unless people start modifying stuff.

Do you want to maximize MPG? Get the 3.21. Do you want to maximize towing? Get the 3.92, but remember to keep your payload up as well (IE don't buy heavy features like ORG/ram boxes/panoramic sunroof, otherwise you still can't tow more than the 3.21).

If you don't really care too much about either, just buy whatever comes in the truck you want, there is literally no reason to worry about what gears you have unless you care very strongly as to either MPG or towing.