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

[BowDown]

I think you're comparing 7 vs 7. Read what I wrote again:
"Put the 3.21 in 7th and it's exactly the same final gear ratio as the 3.92 in 8th".

Which it is. The number of 0.08 is the difference between 3.21 in 6 vs 3.92 in 7. Which means, more than likely, that at the same speed on the highway where the 3.21 is in 6th, the rpm will be equivalent to a 3.92 in 7. Same speed, same rpm, just different gear.

The same holds pretty much true across the last 5 gears, very minor differences.

The major differences are in first/second, and when the 3.92 has no more gears left but the 3.21 can then upshift to 8. If you're rock crawling a lot, by all means get the 3.92.

The point of my posts on all this is that the 3.92 is far less effective than some people think, because the 8 speeds give you so many gear options that you will be able to find overlapping gear ratios for the same rpm, just in a different gear. The numerical gear you are in is irrelvant, what matters is gear ratio at a given rpm, and that's my point; 6th (in the 3.21) and 7th (in the 3.92) have the same final gear ratio, so they will be at the same rpm at the same speed, giving you equal amounts of power at that speed, it doesn't matter that the one truck is in 6th and the other is in 7th.

I agree, its not complicated and IMO, the 3:92's advantages end after 2nd gear.

 

[silver billet]

I agree, its not complicated and IMO, the 3:92's advantages end after 2nd gear.

Example: both trucks are at a dead stop, but have 6000 pounds behind them. They take off in first gear.
The 3.21 has a 16.05 final gear ratio.
The 3.92 has a 19.6 final gear ratio.

Obviously the 3.92 pulls harder .... until, it reaches (say) 15 mph and it has to upshift. It runs out of RPM so it has to upshift, no choice. It drops into second gear. However, the 3.21 is still in first, because it has a taller gear ratio in first, it doesn't run out of RPM's as quickly so it stays in first longer.

At that point, just after the 3.92 upshifted but before the 3.21 has to upshift, we have this scenario at 15 mph:
The 3.21 is in first, 16.05 gear ratio.
The 3.92 is in second, 12.54 gear ratio.

Now it's the 3.21 which is actually pulling harder, especially considering the RPMs are higher and probably making close to peak HP/Torque.

Of course, the trucks continue accelerating and then the 3.21 has to upshift too, so the advantage swings back to the 3.92 the second that upshift occurs. Eventually though, the 3.21 just ends up being 1 numerical gear behind, but at the same RPM and gear ratio.

Basically though, the more gears you have (8, 10+), the less important it is to worry about rear end because the transmission can space out the gears for you. If this was our grandpa's 3 or 4 speed transmission, yeah, much much more important in that case.

(and yes the 15 mph was pulled out of thin air, I'm too lazy to do the math and find the likely shift points, but the exact mph is not important, the point holds whether it occurs at 10/15/20 mph etc.)

 

[BowDown]

Example: both trucks are at a dead stop, but have 6000 pounds behind them. They take off in first gear.
The 3.21 has a 16.05 final gear ratio.
The 3.92 has a 19.6 final gear ratio.

Obviously the 3.92 pulls harder .... until, it reaches (say) 15 mph and it has to upshift. It runs out of RPM so it has to upshift, no choice. It drops into second gear. However, the 3.21 is still in first, because it has a taller gear ratio in first, it doesn't run out of RPM's as quickly so it stays in first longer.

At that point, just after the 3.92 upshifted but before the 3.21 has to upshift, we have this scenario at 15 mph:
The 3.21 is in first, 16.05 gear ratio.
The 3.92 is in second, 12.54 gear ratio.

Now it's the 3.21 which is actually pulling harder, especially considering the RPMs are higher and probably making close to peak HP/Torque.

Of course, the trucks continue accelerating and then the 3.21 has to upshift too, so the advantage swings back to the 3.92 the second that upshift occurs. Eventually though, the 3.21 just ends up being 1 numerical gear behind, but at the same RPM and gear ratio.

Basically though, the more gears you have (8, 10+), the less important it is to worry about rear end because the transmission can space out the gears for you. If this was our grandpa's 3 or 4 speed transmission, yeah, much much more important in that case.

(and yes the 15 mph was pulled out of thin air, I'm too lazy to do the math and find the likely shift points, but the exact mph is not important, the point holds whether it occurs at 10/15/20 mph etc.)

I have a older Gen 3:92 truck, i purposely looked for a 3:21 truck this time, 3:92 was a deal killer for me. Like you, I did the math and its a 20% difference in RPM and thats huge on the highway, secondly, the 8spd ZF trans negates a lot of the 3:92 advantage I've had both and I'll take the 3:21 all day every day and the 18 MPG city that I'm getting

 

[Savage_Doc]

Excellent information! Thanks!

 

[njt07]

To make sure I understand, (BLUF) get the 3.21 cause it and the 3.92 are virtually the same after that initial grunt in the 1st gear?
Gonna go look up the differences between the two for max tow now.

 

[silver billet]

To make sure I understand, (BLUF) get the 3.21 cause it and the 3.92 are virtually the same after that initial grunt in the 1st gear?
Gonna go look up the differences between the two for max tow now.

It's in the extreme ends; 1+2, and 8. But yes once you're moving (city + highway speeds) the 3.21 and 3.92 have almost identically matching gear ratios available, just in a different numerical gear.

I'm not against the 3.92 (at all), it has a place. But too many people get the 3.92 in a well equipped truck and think they can pull 12000+ pounds. That's not how it works. Most 5th gen rams are limited by payload (what you can put in), not what you can pull. 8000 pound trailer is realistically the most amount of weight most 5th gens can pull, regardless of 3.21/3.92, because on average they have about 1300-1500 pounds of payload available (unless you are very careful and make sure not to pick heavy features in your truck). 8000 pound trailer at 15% tongue weight is 1200 pounds of tongue weight on your truck, that's very close to maxing out your truck just by connecting it to your trailer.

So it's about finding a balance. If you buy a barebones tradesman with no sunroof, no etorque, no skid plates, no ram boxes etc, you might have a chance to actually use the 3.92 to full potential. Or if you constantly tow heavy in the mountains, get the 3.92. If you're a more average truck owner and just tow a 6000 pound trailer 2 or 3 times a year, the 3.21 will be fine. What do you do with the truck when you're not towing?

My truck literally spends 98% of running time in 8th gear (rural + highway driving) when I'm unloaded. I pull a 4000 pound trailer for vacations. Will the 3.92 pull that trailer easier in first + second? Yes, but the 3.21 is not struggling either, so for my usage I want the 3.21 due to all that 8th gear driving I do.

 

[Boston]

Most 5th gen rams are limited by payload (what you can put in), not what you can pull. 8000 pound trailer is realistically the most amount of weight most 5th gens can pull, regardless of 3.21/3.92, because on average they have about 1300-1500 pounds of payload available (unless you are very careful and make sure not to pick heavy features in your truck). 8000 pound trailer at 15% tongue weight is 1200 pounds of tongue weight on your truck, that's very close to maxing out your truck just by connecting it to your trailer.

So it's about finding a balance. If you buy a barebones tradesman with no sunroof, no etorque, no skid plates, no ram boxes etc, you might have a chance to actually use the 3.92 to full potential. Or if you constantly tow heavy in the mountains, get the 3.92. If you're a more average truck owner and just tow a 6000 pound trailer 2 or 3 times a year, the 3.21 will be fine. What do you do with the truck when you're not towing?

My truck literally spends 98% of running time in 8th gear (rural + highway driving) when I'm unloaded. I pull a 4000 pound trailer for vacations. Will the 3.92 pull that trailer easier in first + second? Yes, but the 3.21 is not struggling either, so for my usage I want the 3.21 due to all that 8th gear driving I do.

Great answer.

Somewhere here there is a post that includes the weight/payload penalty created by options on the 1500. I need to find it again and can't.

 

[brian42]

3.92 has a published towing capacity of 11K+ lbs and the 3.21 gears have a published towing capacity of 8k+ lbs.

This obviously does not take into account GVWR that silver bullet is talking about. Many people run into this issue where they will exceed GVWR before towing capacity. Exceeding your GVWR and/or towing capacity enters into a legal realm that can end up with you being cited or an accident being your fault regardless of who caused it. Obviously some states are harder than others on this. I live in CA which is usually more conservative than most.

I am very happy with my 3.21 gears. I don't plan to tow often and probably not more than 4-5K lbs when I do.

If I was towing 10K lbs I would have stayed in the 3/4 ton group (my last truck was a 4 ton Ford F-250 diesel) as that would handle the load better than my 5600 lb 1/2 ton no matter what the tow rating is.

 

[Redtrucks]

Looking at ordering a 2021 Ram BH 6’4” box with 20” wheels and trading in my 11 F150 Supercrew with a 5.0 and 3.55 rear end. I live on flat ground but a couple times a year I pull a enclosed sled trailer to Yellowstone and a boat trailer in the summer. Thought that I needed a 3.92 instead of the 3.21 but after going through all these posts l’m having second thoughts. I’ve had at least a dozen pickups over the last 30 years and none have gotten decent mpg until my current Supercrew which is 14-15 winter and up to 18-20 in the summer on the hwy. I use my pickups from going to work, hunting, etc but I would like to get at least in the upper teens for mpg. How much will 20” tires verses the stock 18” effect the engine rpm at hwy speed on each rear end? I live in and near states that the hwy speeds are 75 and 80 mph.

 

[Boston]

Red trucks, it’s overall diameter of the wheel and tire not the rim size that affects mpg, gearing etc.

Stock set ups probably don’t vary in overall diameter.

 

[PicRR]

Thanks for putting in the time for this.

 

[Mtsabo]

This info is great and all but dosent take into account how MDS is affected. I dont know but logic says to me that it will be engaged longer with 3.92 saving fuel closing the gap. Ive found the EPA estimatr to be aversge in my 3.92 truck

 

[silver billet]

This info is great and all but dosent take into account how MDS is affected. I dont know but logic says to me that it will be engaged longer with 3.92 saving fuel closing the gap. Ive found the EPA estimatr to be aversge in my 3.92 truck

So the point you're suggesting is that since the 3.92 revs higher in 8th, and makes more power due to revving higher, MDS can stay active longer. That's a possibility. However, does it offset the fact that you're revving 400 to 500 RPMS (or whatever amount it is) higher at the same speed (mph)? My belief is no, lower RPMs from the 3.21 is still more effective overall, even if MDS is used less often.

For example: I have gotten 24 to 25 mpg consistently on a 3 hour trip (several times making the same trip), with MDS disabled the entire time (I permanently disable it because I hate how it works and jerks and sounds). Assuming MDS is somewhat effective, the same trip might have given me 25 to 26 mpg with it enabled. I have yet to see anybody with a 3.92 post those kinds of numbers.

 

[Mtsabo]

So the point you're suggesting is that since the 3.92 revs higher in 8th, and makes more power due to revving higher, MDS can stay active longer. That's a possibility. However, does it offset the fact that you're revving 400 to 500 RPMS (or whatever amount it is) higher at the same speed (mph)? My belief is no, lower RPMs from the 3.21 is still more effective overall, even if MDS is used less often.

For example: I have gotten 24 to 25 mpg consistently on a 3 hour trip (several times making the same trip), with MDS disabled the entire time (I permanently disable it because I hate how it works and jerks and sounds). Assuming MDS is somewhat effective, the same trip might have given me 25 to 26 mpg with it enabled. I have yet to see anybody with a 3.92 post those kinds of numbers.

View attachment 67318

I was just suggesting it helps close the gap. I know their not equal

 

[Rebelguy2020]

Thank you for posting. I do have a comment/question, my Rebel has the V6 3L Diesel with 3.92 final drive, the ratios will remain the same for 5.7 Hemi but is the shifting done at different RPM? The max torque of 480 lb/ft is at 1600 rpm for the diesel compared to 410 lb/ft at 3950 rpm for the 5.7 Hemi. Before my new 2020 Rebel I had a 2011 Ram outdoorsman with the 5.7 Hemi and the 3.92 final drive but it had the 5 speed (apparently with a step down other second gear) transmission, it did rev a lot between gears when towing. My new Rebel tows with such ease compared to my old truck, a combination of more torque and more gears, I did not try the new Rebel with the Hemi, so I cannot compare it to mine. Is there such a graph to compare the Hemi to the Diesel transmission shift pattern? The programming for both engine/transmission must be very different.

 

[brian42]

Yes, the shifting will be different. The powerband is different on a diesel and the programming will be written to take advantage of that. Plus I don't think you have a 7000 RPM redline like the 5.7L.

My last truck was a Ford diesel that redlined at 3400 RPM.

 

[BowDown]

This info is great and all but dosent take into account how MDS is affected. I dont know but logic says to me that it will be engaged longer with 3.92 saving fuel closing the gap. Ive found the EPA estimatr to be aversge in my 3.92 truck

MDS engagement is based upon engine load determined by the MAP sensor and TPS. I can engage MDS driving through my neighborhood under 25 mph.
A 3.21 truck has 20% less gear than a 3.92 truck and therefore turns 20% less rpms which is 500-600 ram. That 5-600 ram makes a huge difference in fuel economy so no, a 3.92 truck isn't going to get the same mileage as a 3.21 truck.

 

[silver billet]

Also on one of the other forums, somebody posted that a member there did a log of fuel going into each cylinder, and when MDS was active, the fuel for 4 cylinders shut off (like we'd expect), but the fuel in the 4 other cylinders shot up to almost double what it was before MDS kicked in, thereby canceling out. I don't know how true this is. But I do suspect MDS is very ineffective and is saving next to nothing.

 

[BowDown]

Also on one of the other forums, somebody posted that a member there did a log of fuel going into each cylinder, and when MDS was active, the fuel for 4 cylinders shut off (like we'd expect), but the fuel in the 4 other cylinders shot up to almost double what it was before MDS kicked in, thereby canceling out. I don't know how true this is. But I do suspect MDS is very ineffective and is saving next to nothing.

I don't believe that at all. Most fuel cumsiupmtion occurs on acceleration. Consistent speeds, like on the highway, does not require much power, IE, it make take 100hp to accelerate but may only take 40 HP to maintain 65 mph so increased fuel should not be needed. The only time over fueling should occur is when the cylinder needs to be cooled down.
Also, all you're really able to monitor is short and long term fuel trims with a scanner. Unless he was using something like HP Tuners, you're not going to be able to see individual cylinder fueling, only bank to bank.

 

[silver billet]

I don't believe that at all. Most fuel cumsiupmtion occurs on acceleration. Consistent speeds, like on the highway, does not require much power, IE, it make take 100hp to accelerate but may only take 40 HP to maintain 65 mph so increased fuel should not be needed. The only time over fueling should occur is when the cylinder needs to be cooled down.
Also, all you're really able to monitor is short and long term fuel trims with a scanner. Unless he was using something like HP Tuners, you're not going to be able to see individual cylinder fueling, only bank to bank.

He was using HP tuners yes.

But thinking about it logically; if you want to move at 60 mph in 8th gear, you need to make a certain amount of HP/torque to do so. You need fuel and air to make HP. I don't see how you can maintain HP while reducing fuel. His claim that you're just swapping fuel from one cylinder to another makes sense, as then the engine is still making the same amount of HP, it's just "robbing peter to pay paul".

If you reduce fuel, you reduce HP, it's a pretty simple equation to me but I'm not an engineer of that sort. Perhaps there are some light effiencies gained by only firing in 4 cylinders instead of 8, but it certainly can't be exactly half; ie, entering MDS MUST be increasing fuel to the other 4 cylinders that are still active; perhaps it doesn't double what it was before activating, but I"m sure it goes up. Only extensive testing or Ram engineers would be able to determine how much extra fuel is pumped into those 4 cylinders once MDS starts.

And I've mentioned more than once how high I've gotten with MDS completely disabled. I cannot believe I would get higher than 25 mpg even when using MDS. Whatever MDS is doing, it's like in the fractions somewhere, next to nothing.