3:21 rear vs 3:92 gas mileage?

[suicideking]

I don’t think they are making them anymore. I think 2025 production has started.

Correct. 2025 should be available during the second quarter. So might consider getting a 2024 used if I can get a good price. I know the engine is in the Wagoneer, but still pretty new. Probably safer with the 5.7.

 

[Aircommuter]

No, the same final gear ratios are available other than 1st in the 3.92 and 8th in the 3.21. If you're towing in the city or the freeway both trucks will shift identically.



What matters is final gear ratio (transmission * rear axle). Those do approximately equal other than the exceptions mentioned above.

Only for cruising. Starting off with a load like a trailer the engine will be running faster in each respective gear. This will allow the 3.92 truck engine to be closer to its torque rating and it will pull better in each respective gear. The RPM @70 mph is 1950. With stock tires and 3.92. That certainly isn’t going to hurt anything. It certainly doesn’t create more heat. The engine is running easier. As a vehicle goes faster parasitic drag increases, thats going to affect mileage more than engine speed difference.

 

[ramffml]

Only for cruising. Starting off with a load like a trailer the engine will be running faster in each respective gear. This will allow the 3.92 truck engine to be closer to its torque rating and it will pull better in each respective gear.

Your argument is flawed because it assumes both trucks have to be in the same transmission gear at the same MPH. It doesn't. The two trucks will be using different transmission gears to achieve the exact same final gear ratio.

For an analogy: if vendor A (3.21) is selling eggs 3 to a carton, and vendor B (3.92) is selling eggs 4 in a carton, who is selling more eggs (more powerful)? You can't answer that question until you know how many cartons you're buying (tranmission gear).

4 cartons of 3 eggs = 12 eggs total
3 cartons of 4 eggs = 12 eggs total

Same thing with transmission gear and rear axle. They are COMBINED to give you a final gear ratio.

If 3.21 is in 6th and the 3.92 is in 7th, both engines are running the same RPMs and have the same torque multiplication at the wheels.

Yes the 3.92 puts out more torque in 7th vs the 3.21 in 7th, but NOT when the 3.21 is in 6th which is where it will be.

 

[Aircommuter]

I said in each respective gear. A 3.21 truck will not select the gears in attempt to match a 3.92 truck. Why do you think different final drive ratios exist? You need to drive more trucks pulling loads. All manufacturers offer different ratios for different uses. If you want to get real technical you have to include torque converter ratio and locked and unlocked conditions also. If you think they will perform the same you don’t have the engineering knowledge and driving experience.

 

[suicideking]

Also a consideration may be how much you are towing. Everyday, all day, you probably want the 3.92. If it's a daily driver that is rarely towing (my situation), better off with the 3.21.

Though if you're looking for more fun at a green light and don't care about the added fuel expense, go with the 3.92. I'd PREFER the 3.92. Though I'm sure my wife and my checking account would prefer the 3.21.

Bottom line for me is the 3.92 is harder to find in the truck I want, probably a 1500 Big Horn with MSRP around $65K. So will look for a 3.92, but probably end up with the 3.21.

For some perspective, I have a built Jeep for off road with aftermarket gears. It has 37" tires and 5.38 gears, a 4 speed trans (thanks Mopar). I probably wouldn't be unhappy with 5.13, but definitely want more low end torque. Though if it had 5.13 (more suitable for 35" tires), it wouldn't be worth it to pay for a re-gear to 5.38. It's just not enough of a difference. So stepping from one gear set to the next lowest, just doesn't make THAT much of a difference. If you drove the same vehicle with each gear set, I doubt many would be able to tell the difference. WIth the 8 speed, it's even less noticable.

 

[Wild one]

I said in each respective gear. A 3.21 truck will not select the gears in attempt to match a 3.92 truck. Why do you think different final drive ratios exist? You need to drive more trucks pulling loads. All manufacturers offer different ratios for different uses. If you want to get real technical you have to include torque converter ratio and locked and unlocked conditions also. If you think they will perform the same you don’t have the engineering knowledge and driving experience.

The 8 speed locks the convertor in every gear,so that argument is a moot point.It's not like the old days where a convertor only locked in high gear. Once your rolling and past 1st gear with 3.92's,the 3.21's still in first gear have the torque advantage,and in the upper gears,the computor will put both trucks in the engines rpm sweet spot,not the transmission or convertors sweet spot,they'll both be rolling down the road at the same rpm while towing,until you hit 8th

 

[pacofortacos]

3.92's as a general rule for most people will be 2-4 mpg less at 74 mph in my experience.
All stock, I was only able to get 17 mpg @ 74 mph with cruise on the level at sea level.

And that was the best mpg, I have gotten as low as 12 mpg with a fair wind.

 

[ramffml]

I said in each respective gear. A 3.21 truck will not select the gears in attempt to match a 3.92 truck. Why do you think different final drive ratios exist? You need to drive more trucks pulling loads. All manufacturers offer different ratios for different uses. If you want to get real technical you have to include torque converter ratio and locked and unlocked conditions also. If you think they will perform the same you don’t have the engineering knowledge and driving experience.

A 3.21 WILL select the same RPMs that the 3.92 does.

Look at the calculated chart below. When the 3.92 is in 7th gear at 65 mph, it's RPMs (blue cell) are 2194. Now look at the 3.21 columns, the various gears at 65 mph are:

5th/green: 2870
6th/blue: 2191
7th/purple: 1796.

Which gear do you think the 3.21 will be in? Obviously 6th/blue @ 2191, which is exactly 3 rpms less than the 3.92

When your RPMs are "equivalent" and your MPH are equivalent, then so is the torque to the wheels.

 

[mikeru]

3.92's as a general rule for most people will be 2-4 mpg less at 74 mph in my experience.
All stock, I was only able to get 17 mpg @ 74 mph with cruise on the level at sea level.

And that was the best mpg, I have gotten as low as 12 mpg with a fair wind.

Only time I hit 74mph is when I'm passing. Speed limit here is 60 mph or lower.

 

[Wild one]

Only time I hit 74mph is when I'm passing. Speed limit here is 60 mph or lower.

Speed limit on the highway about 5 blocks from me is 110 clics,or basically 70 mph,and on Friday afternoons if you're not doing at least 80,little old ladies are flipping you the bird,lol.
Going up to the track on a long week-end friday afternoon, i've had the cruise plugged in at 94 mph,and i'm still getting passed During daylight hours i've passed cops in the ditch at a 100 mph on that road,with a string of cars on my back bumper,and they don't even blink,after dark it's another story though,after the sun goes down,i don't do much more then 130 clics/80 mph though.

 

[Choupique]

I've never had a truck with the deeper ratios that I didn't like. Have had, 4.30's, 3.73's and 3.42's with 6 speed transmissions. The 4.30's were the coolest but I do tow a fair bit.

In a half ton truck with an 8 speed or 10 speed I dont see where it would matter a whole lot, but ill always recommend the deeper ratio if it is an option. If nothing else, it's easier on the drivetrain and lends itself to longer service life of everything.

 

[ramffml]

If nothing else, it's easier on the drivetrain and lends itself to longer service life of everything.

This particular reason for getting the higher gears is usually trotted out when the performance aspect is thoroughly debunked and the 3.92 guys are left scratching their head trying to justify the gears.

My truck tows my 6000 pounds in 6th gear. Equivalent RPMs would be 7th/overdrive in the 3.92. It's easier on the transmission to tow in direct gear instead of overdrive. The engine doesn't care as RPMs are RPMs.

Taking off from a dead stop? Yes the 3.92 is "easier", but nobody says you have drag race it everywhere. How much time do you spend in first gear? On a 3 hour towing trip my truck will spend < 1 or 2 minutes max, all combined in first gear, but will tow for hours in 6th with occasional downshifts to 5th and when passing in a hurry, 4th very briefly.

And the times you're not towing, well an engine that spins 400 RPMs less over the majority of its lifetime is going to last longer all else being equal.

In other words, there is no golden rule about what's easier, it depends on what you're doing in any given moment. Have you actually found more than 1 or 2 random reports about drivetrain failure (with the 3.21) due to towing heavy? I have yet to see one. The transmission is very stout and the 3.21 guys are drag racing with theirs, this is a complete non issue.

 

[Choupique]

well an engine that spins 400 RPMs less over the majority of its lifetime is going to last longer all else being equal.

The effect on engine life is negligible in the sense of what we're talking about. Engine life is largely simplified down to the total amount of fuel sent through it and today's engines nearly always outlast any pickup truck they're put in. The reduced torque on the stuff behind the engine will generally help all that stuff live longer. Traveling down the road is a work equation and more rpm = less torque = happier parts.

 

[Wild one]

The effect on engine life is negligible in the sense of what we're talking about. Engine life is largely simplified down to the total amount of fuel sent through it and today's engines nearly always outlast any pickup truck they're put in. The reduced torque on the stuff behind the engine will generally help all that stuff live longer. Traveling down the road is a work equation and more rpm = less torque = happier parts.

There's also a downside to your thinking,more rearend rpm also contributes to significantly more heat in the diff fluid,and more wear on the ring/pinion / pinion bearings,and u-joints,as they're all spinning faster. Wear is usually more a factor of the amount of times a component spins in a minute then it is to the load that component absorbs in that same minute , up to it's breaking point of course ,but that isn't really all that big of factor with a 5.7 or 6.4 truck ,usually if they have driveline issues,it's more from wear then it is from load

 

[ramffml]

The effect on engine life is negligible in the sense of what we're talking about. Engine life is largely simplified down to the total amount of fuel sent through it and today's engines nearly always outlast any pickup truck they're put in. The reduced torque on the stuff behind the engine will generally help all that stuff live longer. Traveling down the road is a work equation and more rpm = less torque = happier parts.

400 RPMs over the vast majority of hours it spends in 8th gear is more significant than any stress put on the 3.21 vs 3.92 towing the same load.

I do agree its a nothing burger, all of it; the stress on the drivetrain vs the 400 rpms, it's ALL negligible. You don't hear of any drivetrain issues while towing, and you don't hear on average one engine lasting longer than the other because other variables have much more of a say in the life of the engine.

So don't bring it up then as somehow the 3.92 being the safer option because it's all nonsense. Both trucks are fully capable of towing 8000 pounds and the 3.92 is not going to add any life to the drivetrain over the 3.21. Maintenance and being careful/conservative while towing has infinitely more bearing on this discussion.

 

[Choupique]

So don't bring it up then as somehow the 3.92 being the safer option

Part of what I do for a living is driveline design. The only negative to the deeper ratio is poorer top end light load fuel economy.

Nobody ever regrets getting the deeper gear option in pickup trucks.

 

[ramffml]

The only negative to the deeper ratio is poorer top end light load fuel economy.

And of course the needless extra 400 rpms and wear throughout the lifetime of the engine.

Nobody ever regrets getting the deeper gear option in pickup trucks.

We never hear reports of guys complaining about fuel economy. It never comes up. Never any talk about buying a prius. We're a real happy and satisfied group here.

 

[KKBB]

A 3.21 WILL select the same RPMs that the 3.92 does.

Look at the calculated chart below. When the 3.92 is in 7th gear at 65 mph, it's RPMs (blue cell) are 2194. Now look at the 3.21 columns, the various gears at 65 mph are:

5th/green: 2870
6th/blue: 2191
7th/purple: 1796.

Which gear do you think the 3.21 will be in? Obviously 6th/blue @ 2191, which is exactly 3 rpms less than the 3.92

When your RPMs are "equivalent" and your MPH are equivalent, then so is the torque to the wheels.

View attachment 537857

Nice chart!! I'm gonna ask a question and I'm not trying to be argumentative...just trying to learn. If the ram's are basically the same at 65 mph, will they still be the same at 70, or 75 mph?? If you stayed locked in that gear...or would the 3.92 be at a higher rpm as speed goes up compared to the 3.21?? If they stay the same I can't see a major difference in the gearing, if the 3.92 gears would have higher rpm than the 3.21 at 70, or 75...then I would think the 3.92 would be the better option for towing, but probably not so much for gas mileage.

 

[ramffml]

Nice chart!! I'm gonna ask a question and I'm not trying to be argumentative...just trying to learn. If the ram's are basically the same at 65 mph, will they still be the same at 70, or 75 mph?? If you stayed locked in that gear...or would the 3.92 be at a higher rpm as speed goes up compared to the 3.21?? If they stay the same I can't see a major difference in the gearing, if the 3.92 gears would have higher rpm than the 3.21 at 70, or 75...then I would think the 3.92 would be the better option for towing, but probably not so much for gas mileage.

As long as they are locked in their respective gear, then yes they will both continue to match RPMs as they increase MPH. The chart shows there is a very tiny difference between most gear ratios. But it's a negligble amount in that if you're unhappy towing with one truck then the other truck towing that same amount you'll literally be unable to detect a difference in power with your butt dyno.

There are cases where you can have two different cars or trucks holding steady at one speed, and as they speed up the one car/truck's RPMs will climb quicker, but that's because they are running at a different final gear ratio and the speed/rpms only temporarily match at one given point in time.

In the case of the 3.21 and 3.92, the final gear ratios are approximately equal, so that means their RPMs will continue to match as they increase MPH (with the exception of first/second).

 

[Wild one]

Part of what I do for a living is driveline design. The only negative to the deeper ratio is poorer top end light load fuel economy.

Nobody ever regrets getting the deeper gear option in pickup trucks.

I do. I went from 3.21's to 3.55's,and kick my ass for ever doing it.The truck didn't gain anything at the track,but my milege went down / oil / transmission and rearend temps all went up ,and the biggest differance is the truck lost a good 7 or 8 mph wide open.If i didn't have a pile of money invested in the 3.55 diff,the 3.21 diff's would be going back under the truck. After sticking the 3.55's in the truck,is when i went to dual oil filters to bring my oil temps back to where i liked them more. I have a few buddies with 3.92 trucks,and after an hour on the freeway at 85+ mph,they're wanting to stop or slow down,and let their trucks cool off,as their oil temps are through the roof. A 3.21 truck will stay with traffic on a freeway that's moving at 85+ mph,a 3.92 truck starts to go into meltdown mode trying to stay with traffic on any road where the traffic is running at 85+ mph.