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Old 11-02-2015, 08:13 AM   #1
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Different rear axel ratios

Greetings,

My wife and I are on the verge of purchasing TV. It's a Dodge Ram 1500 with eco-diesel. I have found two to our liking. My question regards the rear axel ratio. One has a 3.21 ratio, the other a 3.55 ratio. Is there much difference between them? I know the 3.55 gets higher gas mileage while the 3.21 has better tow capacity, but is the difference significant enough to definitely purchase one over the other?

Many thanks!

Jim Ford
Kensington, Maryland
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Old 11-02-2015, 08:28 AM   #2
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I think the only EcoDiesel transmission choices are 3.55 which is the default EcoDiesel ratio, and 3.92 which is optional. We noticed 3.55 axles on almost all midwest (flatland) trucks on the lot, and 3.92 axles on most here in Arizona (mountains).

That tells me 3.92 is the best axle choice for towing, especially in a variety of terrain. We have a Ram 1500 Hemi (gas) engine with 3.55 axles, and although decent in most terrain, I would undoubtably buy 3.92 gears next time because of better towing/braking capability in the high country.

It's the other way around with fuel mileage, 3.21 is the best, 3.55 not so good, and 3.92 the worst. But if you're towing or driving in the mountains, the 3.92 gears may do better because there is less transmission searching to find the right gear.
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Old 11-02-2015, 09:01 AM   #3
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Thanks, Doug. I should mention we're looking to purchase a 27' Serenity. While we may travel across the west at some point, most of our RVing will be east of the Mississippi.
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Old 11-02-2015, 09:20 AM   #4
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I think the difference of 3.55 gears and 3.92 is 1 mpg and about $250 option. And with the extra shifting and poorer compression braking of the 3.55 when towing, it's probably a draw.

We have traveled through much of the East as well as the West, and 3.92 gears would have been very nice to have. It would have been an easier towing experience for the truck. Only in the middle states would I choose 3.55 axles on these trucks if used for towing, and only then if I never planned to tow anywhere else.
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Old 11-02-2015, 09:23 AM   #5
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Originally Posted by Jamesbford View Post
Thanks, Doug. I should mention we're looking to purchase a 27' Serenity. While we may travel across the west at some point, most of our RVing will be east of the Mississippi.
We have the 3.55 in our F150 and it works well pulling our 25'. Just curious; have you looked at GVW of the 27' Serenity and max tow capacity of the Eco Diesle from Dodge? Seems to me, your right at the limit of this engine with a 27'? I was also considering this engine when it first came out. There are a few good articles out there talking about towing including this one from 4 Wheeler reviews which talks about lack of power and breaking issues in their test....
2014 Ram 1500 EcoDiesel Review: Towing and MPG Fuel Economy
"Bottom Line: "The EcoDiesel Ram 1500 is not intended to be a heavy-duty truck, or replace a 3/4-ton truck. In fact, if you were looking for max tow capacity from a 1/2-ton, the company would likely steer you into a Hemi V-8 truck with an eight-speed transmission, which has a max-tow rating of up to 10,450 pounds (depending on options). "

I was really hoping this would be "the little engine that could". I want a half ton with power, breaking, great ride, and good MPG....there are several out there, but was hoping this little Eco diesel would be the ticket. Will stay with our F150 Echoboost for now and keep watching...
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Old 11-02-2015, 09:36 AM   #6
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I would second dkottum's advice. I have a 2015 Ram 1500 with the 5.7 hemi, 3.92 rear end and the 8-speed tranny. I love it in the mountains for climbing and decending.

However, I'm still missing the 3.73 rear end I had in my old F-250. I thought it was the best compromise for mostly flat land towing and some occasional hills. Unfortunately, last I checked, Ram doesn't offer that intermediate ratio.
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Old 11-02-2015, 10:28 AM   #7
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I would second dkottum's advice. I have a 2015 Ram 1500 with the 5.7 hemi, 3.92 rear end and the 8-speed tranny. I love it in the mountains for climbing and decending.

However, I'm still missing the 3.73 rear end I had in my old F-250. I thought it was the best compromise for mostly flat land towing and some occasional hills. Unfortunately, last I checked, Ram doesn't offer that intermediate ratio.
I think I would also, "third" dkottum's advice.
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Old 11-02-2015, 11:13 AM   #8
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Not to rain on anybody's technical parade here, but to the OP, I suggest you do some more reading and research on how powertrains ( engine, and specifically the torque the engine can develop ), transmissions and their ratios of each gear, and final drive ( differential ratio ).
Because what has been said so far in this discussion has only talked about the differential ratio. Without knowing the ratios of all the gears in the transmission, you are not getting a valid overall picture of how the truck will "pull".
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Old 11-02-2015, 11:19 AM   #9
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And I'm failing to understand axle ratio recommendations from a gas engine applied to any Diesel engine. Gotta look at a bit more specs and get actual similarly rigged rigs owners comments.
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Old 11-02-2015, 11:23 AM   #10
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Transmissions are a constant, only one choice. Rear ratios are not, different choices, so that is where the The decision is. Go with the one that pulls best, the mileage is not that much different. If you want incredible fuel mileage, get a small car and motel it.
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Old 11-02-2015, 11:42 AM   #11
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I would think for towing the 3:92 rear end gears would be a little better than the 3:55 gears.
In my mind the 3:92 gears are for towing and the 3:55 gears are for economy.
My Tundra only has 1 transmission choice and 1 rear end gear- 4:30-
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Old 11-02-2015, 11:48 AM   #12
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....and for instance, some people tow successfully with a Nissan Titan. It too only has one transmission choice. And it has 2.93 rear gears.
Without knowing and discussing the individual ratios of each gear in the transmission, we simply do not have the entire picture.
As to the suitability of the Ram Ecodiesel, if we want to keep this simple, I agree with our friend above from Mississippi, that given those two choices with that truck, I'd go 3:92.

Now is about when the you-know-what is going to hit the fan about the tongue weight of the trailer vs the available payload of that truck !
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Old 11-02-2015, 12:13 PM   #13
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When I discussed with my Ram dealer which truck was recommended for towing a 25' and longer RV, he recommended the Big Horn with a 5.7L gas Hemi V-8 and the 3.92 axle ratio.

Be sure it has a transmission cooler, or install one after its purchase.
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Old 11-02-2015, 12:27 PM   #14
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I have a 2015 5.7 Ram with 8 speed tranny. I spent 5,000 miles out west mountain driving this summer. Same tranny in the Eco diesel. Best 1/2 ton truck tranny ever owned and I am a diesel biggot. I had a choice of the stock 3.21 or optional 3.92 (pulling 7,300 pound fully loaded 25FC rear birth). I wish the 3.55 was available, but not, didn't want 3.92. I had NO issues or regrets with the 3.21 with the 8 speed. 3.55 would had been perfect. I would think you would be fine with the 3.55 and the diesel. PS: pulling is not going to be an issue for you it's going to be your GCWR.
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