Difference in gas mileage between the 3.21 and 3.92?

[Gmoosevt]

My 2019 3.21 geared 1500 DT Laramie crew cab short bed Sport with the 5.7, 26 gal tank (less weight), no ETorque, no ride height adjustment, 18” Mopar wheels with A/T tires (stock diam.) and a cat back exhaust has averaged 17.3 mpg over its life (on the computer anyway) with not quite 12,000 miles on the odo. I generally use 87 octane, but about every 3rd or 4th fill-up I’ll spring for 89. I live in Vermont, where it is all hills and mountain driving, and a lot of dirt roads. I occasionally tow a 4500lb camping trailer, a 3000 lb bass boat, and a 3000lb capacity utility trailer (with 1 ton of wood pellets, or the month’s garbage to the dump). I often take the truck to do work at my hobby farm up a long dirt road at 1000 ft altitude. My home is 3 miles, all up hill, from the main road and town. So far, my longest trip has only been about 120 miles one way. The speed limit on VT highways is 50 mph, except for one small part of Rt 7 and the one interstate on the Far East side of the state, and up into the Northwest part of the state (that mostly MA, NY, and CT drivers use). I let the truck’s MDS kick in when it wants to. The MDS makes my Borla-S exhaust sound crappy, but I can see the mpg difference on the instantaneous mpg gauge, which appears to be 3-5 mpg improvement on a flat road at 50 or 55 (only Granny actually drives at the 50 mph VT speed limit). I flick on the tow-mode when I’m towing. At 55 mph the truck is almost always in 8th gear, even on gradual grades. On steeper grades it downshifts. My mpg’s definitely improved after about 5000 miles, and the first oil change. Anyway, that’s my real-life mpg situation... for what it’s worth.


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[th365thli]

Wanderer... you seem to be mired in justifying your choice of the 3.92 gear rear. That’s fine. Glad you like it. To each his own. We all tow different stuff at different frequencies, and maybe some want to get off the line quicker than others. However, I might be wrong, but I think the original question was ‘which one gets the best gas mileage?’ The answer to that is simple, either by math, by logic, or by real world testing for the average driver that only tows his boats or trailers every once in a while: the 3.21 gear gets better gas mileage.... as attested to by many, many owners on this very forum (i.e. the ‘real world’). I haven’t heard any Rebel owners (all 3.92 gears of course) bragging about the gas mileage they get.... albeit they have moderately bigger tires and are an inch higher - which kills mpg’s as well. Secondly, why do you thing FCA chose the 3.21 as the standard rear for the truck that all the EPA mpg testing was done on? (that ‘logic’ answer is pretty obvious). And finally, from the ‘math’ perspective, one only needs to think about two identical trucks - but one with 3.92 gear set, and one with 3.21 - driving 55 mph on a flat highway, in identical conditions. Both will be in 8th gear (in identical trannys). The 3.21 will get better mileage by the predominance of lower friction losses from the engine running at lower rpm. Sure, there are many other factors involved, but engine friction is the dominating difference, followed probably by tranny fluid pressure and flow losses, followed by rear gear friction. There are 20 other friction losses involved as well, but almost all of those will be identical for the identical trucks and driving conditions: air friction and rolling mass friction being the biggest. But the point is: it is not debatable - for everyday driving - mixing urban, rural, highway, and occasional towing - the 3.21 is going to deliver better average mpg. Will it tow an 8000 lb horse trailer through the Catskills or over the Rockies as well? Nope. Will it beat the 3.92 in a quarter mile? probably not. Is it better for a weekend in Moab? Nope. So, if off-reading, heavy towing, and hole-shots are important to you, than luckily there is a 3.92 gear set to fit your needs.... but please don’t argue that it gets just as good gas mileage- because it doesn’t.


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Actually I think you misinterpreted him. He's trying to explain how 3.21 is better for gas mileage. It's the other guy who is throwing out some outlandish claims like "3.92 is better from 0-80 mph"

 

[silver billet]

Wanderer... you seem to be mired in justifying your choice of the 3.92 gear rear. That’s fine. Glad you like it. To each his own. We all tow different stuff at different frequencies, and maybe some want to get off the line quicker than others. However, I might be wrong, but I think the original question was ‘which one gets the best gas mileage?’ The answer to that is simple, either by math, by logic, or by real world testing for the average driver that only tows his boats or trailers every once in a while: the 3.21 gear gets better gas mileage.... as attested to by many, many owners on this very forum (i.e. the ‘real world’). I haven’t heard any Rebel owners (all 3.92 gears of course) bragging about the gas mileage they get.... albeit they have moderately bigger tires and are an inch higher - which kills mpg’s as well. Secondly, why do you thing FCA chose the 3.21 as the standard rear for the truck that all the EPA mpg testing was done on? (that ‘logic’ answer is pretty obvious). And finally, from the ‘math’ perspective, one only needs to think about two identical trucks - but one with 3.92 gear set, and one with 3.21 - driving 55 mph on a flat highway, in identical conditions. Both will be in 8th gear (in identical trannys). The 3.21 will get better mileage by the predominance of lower friction losses from the engine running at lower rpm. Sure, there are many other factors involved, but engine friction is the dominating difference, followed probably by tranny fluid pressure and flow losses, followed by rear gear friction. There are 20 other friction losses involved as well, but almost all of those will be identical for the identical trucks and driving conditions: air friction and rolling mass friction being the biggest. But the point is: it is not debatable - for everyday driving - mixing urban, rural, highway, and occasional towing - the 3.21 is going to deliver better average mpg. Will it tow an 8000 lb horse trailer through the Catskills or over the Rockies as well? Nope. Will it beat the 3.92 in a quarter mile? probably not. Is it better for a weekend in Moab? Nope. So, if off-reading, heavy towing, and hole-shots are important to you, than luckily there is a 3.92 gear set to fit your needs.... but please don’t argue that it gets just as good gas mileage- because it doesn’t.


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Agreed, not sure why think I'm arguing against you? 3.21 maximizes mpg, 3.92 maximizes towing power; I've said that from the start. And as you can see from my sig, I do own the 3.21, not the 3.92

However, as the chart above you shows, when the 3.21 is in gear 3 - 7, it's extremely close to final gearing in the 3.92, just 1 gear offset/behind. So that means that it should pull bigger loads very well once you're up to city/highway speeds, it's just getting off the line that the 3.92 will always out pull the 3.21.

 

[Boston]

I'd love to see someone like TFL get their hands on two similarly spec vehicles except for the final drive ratio and put them through their Ike towing and 100 miles mpg test runs side by side, once towing and once not towing.

 

[BigD]

I'm averaging 19 mpg with my 3.92 gears.

 

[2wd]

Which is the better tow vehicle? A Toyota Tundra with brawny 4.30 rear end, or a Ram 1500 with puny 3.21?

Yep it's Toyota of course.

Wait what? The 3.21 actually provides equal or greater pulling power in all gears? WTF!?!?

The ZF 8 speed is a game changer.

 

[Boston]

Which is the better tow vehicle? A Toyota Tundra with brawny 4.30 rear end, or a Ram 1500 with puny 3.21?

Yep it's Toyota of course.

Wait what? The 3.21 actually provides equal or greater pulling power in all gears? WTF!?!?

The ZF 8 speed is a game changer.

The rear end ratio is just part of the story.

 

[Blais Cycle]

What is involved in changing from the 3.92 gear to a 3.21 on my 2021 Bighorn 5.7L Etorque 4X4

 

[Davidbt]

What is involved in changing from the 3.92 gear to a 3.21 on my 2021 Bighorn 5.7L Etorque 4X4

From what I've read on here, you pretty much have to change the front axle. The gears can't be changed easily. Hope that helps.

 

[theblet]

What is involved in changing from the 3.92 gear to a 3.21 on my 2021 Bighorn 5.7L Etorque 4X4

Trade it in on a truck with 3.21s

 

[Davidbt]

Trade it in on truck with 3.21s

That's what I should have added

 

[6of36]

What is involved in changing from the 3.92 gear to a 3.21 on my 2021 Bighorn 5.7L Etorque 4X4

A lot of work and a lot of money. The cost would take longer than the life of the truck to recoup.

 

[Mountain Whiskey]

What is involved in changing from the 3.92 gear to a 3.21 on my 2021 Bighorn 5.7L Etorque 4X4

Buy a Prius, a Sawzall and three packs of demo blades. Unbolt the hatch and proceed to square everything off. You will have a 3.21 Truck ready to go!

Don't forget new badges from eBay!

 

[bigdodge]

another gear post that just will not die!

 

[retgi]

I just got a 23' limited 3:21 gears with etorque MDS and seeing about 21 mpg at 65 mph. Will it get any better than this with some break in or is this about it?
I've always been a Ford guy and loved the 2.7 ecoboost with plenty power, averaged 23 mpg at 75 mph but decided to take a chance on this Ram since getting a good amount knocked off
the price. I liked getting this deal and having all the bells & whistles except for a sunroof which I don't need or want here in Florida and the larger gas tank.

 

[azzx]

What is involved in changing from the 3.92 gear to a 3.21 on my 2021 Bighorn 5.7L Etorque 4X4

Seriously!!!!, Do your homework first before you buy a "truck"
Sell your truck and buy something else. This thing is a 6000lb truck - designed to carry a lot of stuff and guzzle gas.

 

[azzx]

another gear post that just will not die!

Yes - it is amazing how people don't do any due diligence before buying a product!

 

[Willwork4truck]

I just got a 23' limited 3:21 gears with etorque MDS and seeing about 21 mpg at 65 mph. Will it get any better than this with some break in or is this about it?
I've always been a Ford guy and loved the 2.7 ecoboost with plenty power, averaged 23 mpg at 75 mph but decided to take a chance on this Ram since getting a good amount knocked off
the price. I liked getting this deal and having all the bells & whistles except for a sunroof which I don't need or want here in Florida and the larger gas tank.

My previous 2019 hemi non ET 3.21 with 5’7” bed and 4x got the same as you reported (lie-o-meter) at 65 mph occasionally saw 22.
My current 2023 hemi ET 3.92 gets 18 Mpg. The gearing is the difference. Same roads, same speed, same old man driver.

 

[theblet]

I just got a 23' limited 3:21 gears with etorque MDS and seeing about 21 mpg at 65 mph. Will it get any better than this with some break in or is this about it?
I've always been a Ford guy and loved the 2.7 ecoboost with plenty power, averaged 23 mpg at 75 mph but decided to take a chance on this Ram since getting a good amount knocked off
the price. I liked getting this deal and having all the bells & whistles except for a sunroof which I don't need or want here in Florida and the larger gas tank.

You’re actually doing better than 90% of us on mpg.

 

[Slutzk]

My truck is in signature, but i have mine as a 3.21 and my buddy has a Laramie as 3.92 with the same exact size truck, size tire and suspension level. we both are around 14-15 mpg. so id say no not much to notice really.