2023 5.7L V8 HEMI MDS VVT eTorque Engine 3.21 Rear Axle Ratio

[Dodge 1500 4X4]

One thing here aren't the engine RPM's higher on 3.92 verses 3.21 through the 8-speed shifting higher RPMs consume more fuel in my book.

 

[ramffml]

One thing here aren't the engine RPM's higher on 3.92 verses 3.21 through the 8-speed shifting higher RPMs consume more fuel in my book.

Yes. See the chart below. If both trucks are 6th gear at 65 mph, then the 3.21 will be at 2191 rpms and the 3.92 will be at 2675 rpms.

But the key point to remember is that both trucks won't be in the same gear anymore at the same time. They can't be, because the same reason that allows the 3.92 to take off easier from a stop (deeper gear) is the same reason why it will hit redline first, and the same reason that will force it to upshift into second well before the 3.21 does. Then eventually we reach a point where the 3.92 is basically at the same RPMs as the 3.21, just in one numerical gear ahead. But again, numerical gear doesn't equal final gear ratio. At 65, we can see they both have the ability to be at 2190-ish rpms at 65 mph.

So if both trucks are under the same load and accelerating at the same rate, the point where the 3.21 saves fuel is when it finally shifts into 8th. Because at that point the 3.92 has no more gears left to upshift (and lower the RPMS).

So the 3.21 only saves fuel when it's in 8th gear. Otherwise there is no significant difference on average, unless we want to calculate the exact speeds where one truck is running a tiny bit lower rpms vs the other; but then you drop 5 mph and one truck upshifts and suddenly it's using more fuel, so it becomes a wash in the city. But the 3.21 on the highway in 8th gear, it's running 400-ish rpms lower than the 3.92, and that will add up quite a bit over the life of the truck.

 

[Dodge 1500 4X4]

But the 3.21 on the highway in 8th gear, it's running 400-ish rpms lower than the 3.21, and that will add up quite a bit over the life of the truck.

Also, better on the valve train!!!

 

[KWal]

Great thread! Thanks for the valuable information and insight.

 

[crash68]

See the chart below. If both trucks are 6th gear at 65 mph, then the 3.21 will be at 2191 rpms and the 3.92 will be at 2675 rpms.

You can point at your speed chart all you want but I've seen multiple times people stating the the lower gear ratio will tow better. This has been with multiple different powertrains where the only difference was the rear axle ratio is different (engine, trans, body, etc. all the same). Discussions have turned into arguments over the subject with ones pointing at their charts and the others going by real world experience. The chart is great for showing RPMs but it doesn't factor in applied forces.
Put this into the thought process, changing the rear axle ratio changes the amount of force applied over the rate of feed. As the gear ratio is lowered the amount of torque goes up over a shorter distance. Keep in mind towing is a constantly changing dynamic load.

 

[ramffml]

You can point at your speed chart all you want but I've seen multiple times people stating the the lower gear ratio will tow better. This has been with multiple different powertrains where the only difference was the rear axle ratio is different (engine, trans, body, etc. all the same). Discussions have turned into arguments over the subject with ones pointing at their charts and the others going by real world experience. The chart is great for showing RPMs but it doesn't factor in applied forces.
Put this into the thought process, changing the rear axle ratio changes the amount of force applied over the rate of feed. As the gear ratio is lowered the amount of torque goes up over a shorter distance. Keep in mind towing is a constantly changing dynamic load.

The only thing that matters is final gear ratio. If two trucks have the same final gear ratio, then while they are using that ratio they will be at the same RPMs at a given speed, putting the same torque down to the wheel.

This is a mathematical certainty, there is no room for subjective opinions on what feels better.

I'm a huge audiophile, and spend just as much time on audio forums. If you've been there for more than a week, you'd see firsthand the amount of stuff people can convince themselves of. Cable lifters, $800 power cords that are 2 feet long but somehow improve the signal between the power plant and the speakers etc etc. We're all guilty of this at some point, myself included, we change something and we want to feel the improvement or justify the purchase or whatever it might be, so then we "feel" the difference.

The thing with gears are, we know how they work and how torque is multiplied. That's why I used the egg example, you may as well convince me that one of those 2 guys is giving me more eggs than the other.

If I'm wrong on the math/gear ratios, then show me your math that proves I'm wrong. This is a simple math problem, either 4 * 3 doesn't equal 12, or I'm using multiplication instead of subtraction, or something like that ... if I'm wrong, you should be able to demonstrate it using math because we're not going to get anywhere further without math.

Put this into the thought process, changing the rear axle ratio changes the amount of force applied over the rate of feed.

Of course it does. What you're missing, is that there are TWO variables; transmission gear, and axle gear. If you change the axle gear while also changing the transmission gear you can cancel the changes out 100% which is what is going on here very specifically with the 3.21 vs 3.92.

If the argument was 3.55 vs 3.92, the math shows that both trucks would have different behaviour because they'd never be at the same RPMs and therefore they'd never be putting out the same torque at the same MPH.

This is a very specific case of the ZF 8 speed transmission combined with 3.21 vs 3.92, where the differences line up so neatly that a simple downshift by the 3.21 puts you at the same spot as the 3.92 which is one gear ahead.

If you were to change the transmission with differnet gears, or one of the axle ratios became 3.55 or 4.10, it'd be a completely different discussion.

 

[Ramfanski]

This link might be useful.

The 3.92 will be able to do the 0-30 and 0-60 tests with much more weight due to its gearing in the first couple gears of the 8 speed transmission. That is the reason it has a few thousand pounds advantage in tow weight ratings. The other tests, would be equivalent as the 3.21 would just run in a lower gear.

No matter what gears you have, the trucks are very capable of towing less than 8000 pounds easily as long as you set up correctly. I personally would not go over that regardless, at least not on a regular basis. That is just me though.

 

[David James]

I tow a traile

i would not want to pull any travel trailer without a2500. I would rather be overly qualified then slightly under qualified

i have both the 3.92 is faster off the line but for every day driving3.21 is better. Honestly at 70 mph couple hundred rpm difference just run it in tow haul and in 7 gear

Fully agree. The 3.21 rear end is worth almost a mpg on the road over a 3.55 and more over a 3.92. Quieter and still plenty of beans. Tow/haul is there for a reason. 8 speed is there for a reason. I believe anything over 5k ought to be behind a medium duty like a 2500. Much safer.

 

[NotSoFast]

I dunno much about gears, but I tow a travel trailer a lot. And lots of that with mountain grades and high winds and I live at 7000 feet. My 1500 eco-diesel towed ok, but hated having to accelerate on an uphill freeway ramp. Then it tossed a rod through its case at 35000 miles, and the dealer couldn't get a replacement engine. So I traded it in for a 2500 with a Cummins. What a difference! It is rock solid in high winds, it has enough torque to climb trees, it stays cool when towing in the heat, and engine braking and exhaust braking are not a comparable to the eco-diesel. The downside is that I use more diesel, more DEF, and maintenance costs more.

 

[yrraljguthrie]

I'm assuming it's a 20-foot trailer by its name. That truck will pull that trailer 70mph all day long with no trouble. No need for a 2500 or 3.92.

 

[Tulecreeper]

I'm assuming it's a 20-foot trailer by its name. That truck will pull that trailer 70mph all day long with no trouble. No need for a 2500 or 3.92.

I must respectfully disagree. Towing anything at 70 MPH, that's trouble right there.

 

[Huliodude]

Reviving an old thread as I was researching the pros and cons of having a 3.21 gear vs 3.92. Several members on the forum have said it only makes a difference accelerating from a dead stop or cruising above 65mph, as the 8spd trans will choose the appropriate gear for the job at hand at any given speed during cruising. For example, at 40mph a truck with a 3.92 will cruise in 8th gear, where the same truck same scenario but with a 3.21 will be in 7th, and both trucks will be at the same rpm using the nearly the exact same effective gear ratio.

While it’s true that both trucks are at the same rpm and same gear ratio, just a different transmission gear, the similarities end there.

A truck with a 3.92 can tow better and accelerate harder at any speed vs the 3.21. Here’s a couple of reasons why:

Accelerating from 0-40 mph, a 3.92 geared truck will use all 8 gears, while a 3.21 geared truck will only use 7.

Each gear in a 3.21 geared truck is effectively longer because of the rear gear. Using only 7 gears to get from 0-40 mph means the truck spends more time in each gear. Longer time in each gear means longer time for your engine to sweep through the rpm range. While your engine is sweeping through the lower rpm range, it’s not at an rpm that makes as much power as when it’s sweeping through the higher rpm range. With a 3.92, every gear is effectively shorter, allowing the rpm’s to climb faster and get your motor to a higher rpm faster, where it can make more power.

The other reason a 3.92 is better for towing and accelerating is because it uses all 8 gears more often, as they are shorter and more ratios allows your engine to be at a more efficient rpm range more often and in more scenarios. This is one of the reasons why manufacturers aren’t selling trucks with 4 spd automatics anymore. Even if 4th gear of old trucks had the same gear ratio as our 8th gear today, it would need to be much longer. A 4spd trans would need very long gears to have its 4th gear match the ratio of todays 8th gear, causing huge rpm drops with each up shift, putting the engine at a mechanical disadvantage during lower rpms, which is inefficient. This is also why GM and Ford developed 10spd autos. More gears gives the engine more opportunity to operate at more efficient rpms more often.

Anyone who thinks a 3.92 vs a 3.21 have the same accelerating and towing capabilities once you’re moving I think is just incorrect.

Now for fuel efficiency, once you’re on the highway and your 3.21 geared
truck can use 8th gear and hold it, it will save gas over the 3.92. Lower rpm means less friction and pumping losses in the engine, saving fuel.

But there is one variable I’m not sure about: MDS. Won’t your engine be able to stay in 4cyl mode more often with a 3.92 vs a 3.21, which could help the 3.92 gear match the fuel economy of 3.21 and even beat it it in some scenarios?

Can anyone who’s had both rear gears chime in on how MDS was affected by it?

Thanks!

 

[Mister Luck]

It’s about maintaining hwy speeds uphill for towing verses fuel economy, what your intended use for your truck when you bought it.

I have MDS but I adjust the throttle response and so the gear selection in how soon it shifts out of final gears you should consider final weight to horsepower ratio , that’s what it ultimately comes down to.
(Wheel size also plays into final gear ratio)

 

[ramffml]

Reviving an old thread as I was researching the pros and cons of having a 3.21 gear vs 3.92. Several members on the forum have said it only makes a difference accelerating from a dead stop or cruising above 65mph, as the 8spd trans will choose the appropriate gear for the job at hand at any given speed during cruising. For example, at 40mph a truck with a 3.92 will cruise in 8th gear, where the same truck same scenario but with a 3.21 will be in 7th, and both trucks will be at the same rpm using the nearly the exact same effective gear ratio.

While it’s true that both trucks are at the same rpm and same gear ratio, just a different transmission gear, the similarities end there.

A truck with a 3.92 can tow better and accelerate harder at any speed vs the 3.21. Here’s a couple of reasons why:

Accelerating from 0-40 mph, a 3.92 geared truck will use all 8 gears, while a 3.21 geared truck will only use 7.

Each gear in a 3.21 geared truck is effectively longer because of the rear gear. Using only 7 gears to get from 0-40 mph means the truck spends more time in each gear. Longer time in each gear means longer time for your engine to sweep through the rpm range. While your engine is sweeping through the lower rpm range, it’s not at an rpm that makes as much power as when it’s sweeping through the higher rpm range. With a 3.92, every gear is effectively shorter, allowing the rpm’s to climb faster and get your motor to a higher rpm faster, where it can make more power.

The other reason a 3.92 is better for towing and accelerating is because it uses all 8 gears more often, as they are shorter and more ratios allows your engine to be at a more efficient rpm range more often and in more scenarios. This is one of the reasons why manufacturers aren’t selling trucks with 4 spd automatics anymore. Even if 4th gear of old trucks had the same gear ratio as our 8th gear today, it would need to be much longer. A 4spd trans would need very long gears to have its 4th gear match the ratio of todays 8th gear, causing huge rpm drops with each up shift, putting the engine at a mechanical disadvantage during lower rpms, which is inefficient. This is also why GM and Ford developed 10spd autos. More gears gives the engine more opportunity to operate at more efficient rpms more often.

Anyone who thinks a 3.92 vs a 3.21 have the same accelerating and towing capabilities once you’re moving I think is just incorrect.

Now for fuel efficiency, once you’re on the highway and your 3.21 geared
truck can use 8th gear and hold it, it will save gas over the 3.92. Lower rpm means less friction and pumping losses in the engine, saving fuel.

But there is one variable I’m not sure about: MDS. Won’t your engine be able to stay in 4cyl mode more often with a 3.92 vs a 3.21, which could help the 3.92 gear match the fuel economy of 3.21 and even beat it it in some scenarios?

Can anyone who’s had both rear gears chime in on how MDS was affected by it?

Thanks!

The short answer is you're wrong, I'm way too tired of this argument to get into details about it but if you actually sit down and calculate the final gear ratio's for these 2 exact trucks you'll see why.

 

[Wild one]

The short answer is you're wrong, I'm way too tired of this argument to get into details about it but if you actually sit down and calculate the final gear ratio's for these 2 exact trucks you'll see why.

Another guy who doesn't understand final drive ratio's,but he did make me laugh with his theories,

 

[Huliodude]

Another guy who doesn't understand final drive ratio's,but he did make me laugh with his theories,

I think I understand them pretty well, however my experience with gear ratios are not scientific, but are real world. I've certainly ridden 15spd bicycles and could feel the difference in resistance on my legs using different gears. I've owned a 5.0 Mustang and swapped the gear from a 3.08 to a 3.73 and certainly felt the difference. In town, there was no difference in MPG, I just used top gear in the transmission more often. On the highway however was another story. I have a KTM 500 where I sometimes change rear sprocket ratios in order to facilitate different types of riding (tight woods vs fire roads for example), and I certainly feel the difference there as well.

I want to get the 3.21 on my new truck, but I think the 3.92 is the way to go for my purposes, even though I rarely tow.

Two RAM 1500 Hemi trucks with the ZF8 speed optioned exactly the same, except 1 has a 3.21, the other 3.92. Here's the claims I'm making:

There is no measurable fuel economy difference at any speed below 60mph.
The 3.92 truck will win a 0-60 race consistently.
The 3.92 will be more responsive on a roll, and will be quicker all around (roll race, from a dig, 40-70mph, whatever)
The only advantage the 3.21 has over the 3.92 is highway cruise speeds, where the motor will turn at a lower RPM in 8th gear and save fuel.

If I never speed on the highway and stick to the speed limits, I think the fuel economy difference will be next to nothing, immeasurable.

And here's one hope I have, but not sure on:

The 3.92 will allow the Hemi to run in MDS mode more often and in more scenarios, saving fuel and easily matching the economy of the 3.21 in almost all cases.

I would appreciated being proven wrong, as I will learn something new.

 

[ramffml]

I think I understand them pretty well, however my experience with gear ratios are not scientific, but are real world. I've certainly ridden 15spd bicycles and could feel the difference in resistance on my legs using different gears. I've owned a 5.0 Mustang and swapped the gear from a 3.08 to a 3.73 and certainly felt the difference. In town, there was no difference in MPG, I just used top gear in the transmission more often. On the highway however was another story. I have a KTM 500 where I sometimes change rear sprocket ratios in order to facilitate different types of riding (tight woods vs fire roads for example), and I certainly feel the difference there as well.

I want to get the 3.21 on my new truck, but I think the 3.92 is the way to go for my purposes, even though I rarely tow.

Two RAM 1500 Hemi trucks with the ZF8 speed optioned exactly the same, except 1 has a 3.21, the other 3.92. Here's the claims I'm making:

There is no measurable fuel economy difference at any speed below 60mph.
The 3.92 truck will win a 0-60 race consistently.
The 3.92 will be more responsive on a roll, and will be quicker all around (roll race, from a dig, 40-70mph, whatever)
The only advantage the 3.21 has over the 3.92 is highway cruise speeds, where the motor will turn at a lower RPM in 8th gear and save fuel.

If I never speed on the highway and stick to the speed limits, I think the fuel economy difference will be next to nothing, immeasurable.

And here's one hope I have, but not sure on:

The 3.92 will allow the Hemi to run in MDS mode more often and in more scenarios, saving fuel and easily matching the economy of the 3.21 in almost all cases.

I would appreciated being proven wrong, as I will learn something new.

Buy a truck with a 3.21. Take out 8th gear, push the remaining 7 up, and add a very low first gear. That's essentially what you do to create a 3.92. You still have 8 gears, but the very deep first gear in the 3.92 has no effect anymore once you shift out of it.

This is not 100% accurate of course, but if you do the math on the final gear ratios you'll see how close that example is and it illustrates why 3.92 doesn't have any advantage beyond first/second.

As for racing, you'll have to ask Wild One but I believe a number of guys do well with the 3.21 because it doesn't need an extra shift to hit 60 mph.

The 3.21 also has an advantage while towing if you tow in 6th gear like I do because that is "direct drive", and at that load/speed/rpms the 3.92 has to be in overdrive/7th to match it. If you want to tow on the freeway in direct drive with the 3.92 you'll be at 2700+ rpms for no reason.

MDS doesn't save you any fuel in the first place. Doubling nothing is still nothing. I've disabled it from day one. However MDS cannot overcome the savings from running 400 rpms lower. All the high MPG reports come from 3.21s.

I couldn't care less what people prefer, if you like the 3.92 for one reason or another then that's the gear for you. But facts are facts, that's all I'm trying to argue are the facts.

 

[ramffml]

As a follow up to the above: if there was any indication that the 3.92 with MDS saved fuel, even if it was the teeniest of tinest of savings, Ram would be all over it like a dirty shirt and would be selling every truck with the 3.92. They would not offer the 3.21. They need every fraction of a percent of savings which is where we get stuff like MDS and watery thin 20 weight oils from. It's simply not true, the 3.21 saves fuel.

 

[Huliodude]

Buy a truck with a 3.21. Take out 8th gear, push the remaining 7 up, and add a very low first gear. That's essentially what you do to create a 3.92. You still have 8 gears, but the very deep first gear in the 3.92 has no effect anymore once you shift out of it.

What you say here is true, however up to 40mph, the 3.92 truck can use all 8 gears where the 3.21 will only use 7. 8 gear ratios available to choose from is better than 7. The motor can run closer to its optimal RPM range for the task at hand when it has more gears to choose from. This is why a 10speed can save even more fuel.

As for racing, you'll have to ask Wild One but I believe a number of guys do well with the 3.21 because it doesn't need an extra shift to hit 60 mph.

Yes I didn't consider the extra shift that may be necessary. It's dependant on your trap speed and how much HP your truck is making. But in all fairness, the ZF8 shifts at lightning speed, and I doubt the extra shift would lose all the advantage you've gained from the very short 1st gear on launch with the 3.92.

The 3.21 also has an advantage while towing if you tow in 6th gear like I do because that is "direct drive", and at that load/speed/rpms the 3.92 has to be in overdrive/7th to match it. If you want to tow on the freeway in direct drive with the 3.92 you'll be at 2700+ rpms for no reason.

So you're saying 6th gear direct drive has less driveline loss than 7th gear. Probably true. Maybe that could save more gas than a 3.92 in 7th doing the same job?? It's a pretty rare scenario when pitted against all the other potential driving situations we get into.

MDS doesn't save you any fuel in the first place. Doubling nothing is still nothing. I've disabled it from day one. However MDS cannot overcome the savings from running 400 rpms lower. All the high MPG reports come from 3.21s.

I don't believe MDS doesn't save fuel. It HAS to save fuel. Running in 4cyl mode during cruise takes advantage of one thing in two ways: Less pumping losses. 4 cylinders needs to work twice as hard as 8 cylinders, causing your throttle body position to be opened further, reducing intake vacuum. High intake vacuum is lost energy. MDS activates when the cylinder being turned off is at bottom dead centre, closing both valves. When that dead cylinder is on it's way back up and compressing the air but not igniting it, the pressurized air pushes the piston back down, and the cycle repeats itself. It's similar to a highly inflated basketball. If you drop it, it will bounce back up, but not quite as high as the point you dropped it from. That's the measurable loss. Keeping that cylinder running when it's not needed creates pumping losses, such as pulling air into that cylinder on every stroke, and pushing that exhausted air out the exhaust valve. All this is negated when the valves are kept closed.

I couldn't care less what people prefer, if you like the 3.92 for one reason or another then that's the gear for you. But facts are facts, that's all I'm trying to argue are the facts.

I appreciate you humouring me and my thoughts. I will visit a dealer and try out the 3.92. I'm betting I'll be sold.

 

[ramffml]

What you say here is true, however up to 40mph, the 3.92 truck can use all 8 gears where the 3.21 will only use 7. 8 gear ratios available to choose from is better than 7. The motor can run closer to its optimal RPM range for the task at hand when it has more gears to choose from. This is why a 10speed can save even more fuel.

The first gear on the 3.92 is very short lived. You're not spending any time in that gear. Again, if there was any possibility that Ram thought the 3.92 saved fuel it would be the default/only option to purchase as there would be no point to the 3.21 then.

Yes I didn't consider the extra shift that may be necessary. It's dependant on your trap speed and how much HP your truck is making. But in all fairness, the ZF8 shifts at lightning speed, and I doubt the extra shift would lose all the advantage you've gained from the very short 1st gear on launch with the 3.92.

So you're saying 6th gear direct drive has less driveline loss than 7th gear. Probably true. Maybe that could save more gas than a 3.92 in 7th doing the same job?? It's a pretty rare scenario when pitted against all the other potential driving situations we get into.

I don't believe MDS doesn't save fuel. It HAS to save fuel. Running in 4cyl mode during cruise takes advantage of one thing in two ways: Less pumping losses.

We all understand the theory on how this works. In practice, MDS only activates when you're throttle inputs are so low that you're basically idling (slight exaggeration but you get the point). Right before MDS activates, the truck is already using no fuel, so trying to save an additional 10% of no fuel still equals no fuel.

This fuel saving from MDS is not larger than the fuel savings you get from running at 400 rpms lower. The 3.21 even with MDS disabled saves more fuel than the 3.92 with MDS enabled.