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Real strong argument to justify your purchase, but saying the 3.92 improves MPG everywhere but open interstate just isn't accurate.

Looking at the above final drive ratio chart, I'd probably choose the 3.21 as well. Running one gear lower there's a slight mechanical advantage over the 3.92 truck. I tow heavy and I'm in no hurry in first gear, so that's a non-issue. Never use OD gears when towing so another non-issue. 3.21 for the all-around win.
But 1st is a mechanical disadvantage with the 3.21s, which is important while towing. And while cruising on the interstate, the 3.21s are either going to be at less RPM in 8th which will make it harder to cruise, or more RPM in 7th which would consume more fuel.

Bottom line is that it's like the age-old 10-speed bicycle analogy.... nobody wants to pedal uphill in the higher gears. It requires a lot more energy to do that. It's actually the lower gearing that gives ease to keeping the wheels rolling.

For fuel efficiency and performance I'll stick with the 3.92s.

Also, if you choose to run larger tires like me that plays into it as well. With my current tires, it's effectively like having a 3.65 axle gear.
 

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Thanks for the lesson in gearing but it's really unnecessary

But by all means, continue trying to convince me 3.92 is ever better for MPG than 3.21.
 

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It seems like a very difficult concept to understand, but 7th with the 3.21 is nearly identical in rpms and torque to the 3.92 In 8th. 6th with the 3.21 is exactly the same as 7th with the 3.92. Your stammers are not correct expect for taking off in first. With the 3.21 tow-haul locks out 8th so it tows the same. Taking off with the 3.21 is again not an issue because the diesel makes a lot of torque at low rpms. However there is 20% more stress on the 3.21 driveline when taking off on a steep hill.

The 3.21 is fine on the highway in 8th when not towing. Again comments from someone who doesn’t own the 2020 Eco 3.21.
 

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I'm in the 3.92 camp, it's a truck to haul loads, if you need to drive at highway speeds empty, get a Prius.
I'd prefer a 4.10, had a V10 ram, it couldn't start from stopped when pulling a 4 horse living quarters rig, useless.
 

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I’m looking to buy a 3rd Gen ED and noticed how many of the ones sitting on the lot are geared with 3.21 gears. Who likes this and who wishes they had the 3.92.

Jeff
I had the 3.21 on a 2015 and loved it. Currently have a 2018 with the 3.92 and hate it and want to trade it. The rpm are just too high on the freeway and mpg at least 2 mpg less with my driving style. Never had trouble towing with 3.21.
 

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I had the 3.21 on a 2015 and loved it. Currently have a 2018 with the 3.92 and hate it and want to trade it. The rpm are just too high on the freeway and mpg at least 2 mpg less with my driving style. Never had trouble towing with 3.21.
3.21 were not made on a 2015 ecodiesel you either has 3.55 or 3.92.
 

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There's a reason the 3.92 gear is a $95 option and is one of the most frequently selected options on the order sheet.

Also, my 8th is not your 7th.

Gear x3.21 x3.92
5.00 16.05 19.60
3.20 10.27 12.54
2.14 6.87 8.39
1.72 5.52 6.74
1.31 4.20 5.14
1.00 3.21 3.92
0.82 2.63 3.21
0.64 2.05 2.51

As you can see there isn't one single place anywhere in the entire gear spread (except 1:1 drive obviously 6th gear w/ 3.21 vs. 7th w/ 3.92) where the two axle ratios overlap. But, the 3.92 will be able to hold 8th far easier on the interstate, while still turning less RPM than 7th gear with 3.21 axle gears would. So, the 3.92 is better everywhere from 0 to 80 miles per hour.

I favor the 3.92 because it takes off easier, accelerates easier, cruises in town easier, and stays in 8th gear on the highway more often. This, in turn, actually improves MPG everywhere except the interstate.
How is Mpg going to be improved when rpm is constantly higher regardless of conditions?
 

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How is Mpg going to be improved when rpm is constantly higher regardless of conditions?
A lower axle ratio is less load on the engine. Just because an engine is spinning xxx rpms doesn't equate to higher fuel consumption, it's the resistance to the pistons going down that raises the fuel consumption. In start/stop traffic a lower axle ratio will do better.
 

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To me it all depends on your usage.

People respond often like the "Blind Men and the Elephant". They know their driving habits and normal patterns. That translates to being a major influence on their opinions. tow heavy, get the lower gearing. Tow light but drive under slower conditions - pretty much anything works. Tow occasionally, light or a little heavy but drive Interstate speeds - probably do better with the higher ratio which would be the 3:21.

Now the little Ecodiesel engine is "little". If you plan on towing much, go the lower ratio (3:92). If you have a big engine like I now have, I would love a higher ( lower numbered) ratio as I never need the pulling power for anything I might pull. Your opinion is most often based on your usage.
 

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I think theres more to this than simply calculating gear ratios ( or final drive; which needs to include tire size/type/etc (which also varies per trim or model); running michelin all seasons vs toyo RTs is not an equal comparison.)).

Like Crash said, I think engine load is actually the largest factor, because thats what alters fueling and timing. Also the way you drive, and how the computer and trans reacts to your driving style. Everyone could own the exact same truck, one guy gets 29mpg, slow rolls the throttle and is never in a hurry. The other guy gets 20 mpg, drives like the modern A-hole on the road, needs to race traffic light to traffic light, and doesn't look up the road to gain speed for hills even though he has 8k in-tow.

And that's what you really can't compare everyone's thoughts or opinions on here, because none are identical and everyone has a "preference".

You need to decide what you actually plan to do with the truck, what your driving style is like, and go from there. Or at least tell us what your plan is so we have a better idea.

You also nee to consider options. Like a locking rear diff. That was a must for me, and it only comes in the 3.92.

Good luck.
 

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I towed a 8500lb 35ft trailer up into the adirondacks, and needed every gear that truck had to offer and wanted a 4.x gear to be honest.

yeah, you can shift down and get to the same outcome unless you're trying to pull out a stump (which i've done!)
 

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You are correct. Missed that. Either way that change alone was worth about 2 mpg at 80 mph highway speeds I typically run.
 

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it's a truck to haul loads
Well, my old Dodge 2500 Cummins was a "truck to haul loads". It's fine and dandy that some people tow 10,000 pounds in a 1500. However the data seems to indicate that most people who own a 1500 truck seldom tow over a few thousand pounds.

My 2016 1500 with 3.92 and GDE gets 1.5 or 2.0 less mpg than the 3.55, and this seems to be the "norm" not towing. But, I've not thought about the "around town" mpg and don't know the answer to that.
 
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