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Discussion Starter #1
Thanks to the posting on the spec build on the Ram site. It's interesting to see how the different options affect tow capacity. etc. One that is a fairly major decision, is the drive ratio.
Understanding 355 has longer legs, better for highway fuel economy.... 392 better for tow capacity, and probably better economy around town.
My criteria for highway cruising, if we go on vacation or a trip, NOT towing, would be to cruise 75 mph.... and I understand both ratios will do this, but not sure what the economy would be with the 392.

I guess other option would be on a trip, to slow down to 70 and be happy at that speed... .

Anyone have thoughts on which ratio 355 or 392, understanding for sure it's based a lot on how the truck will be used, and how the truck will be driven?
Marsman
 

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this was posted on another forum. it is talking about the 3.21 vs 3.92 but the chart showes the rpm differance. i was a little concered but about this too but i think the 3.55 will work for my needs.

at freeway speeds of 70 MPH, the 3:21 axle ratio will spin the engine at about 1500 RPM, the 3:92 axle ration will spin the engine at about 1850 RPM. A faster spinning engine will use more fuel while the engine load is nearly identical
Ratios.JPG
 

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Discussion Starter #3
The chart shows where the motor rpm is at certain speeds, and unless you are towing a lot, the 355 looks to be good middle ground...... with 321 being long, and 392 being steep.
 

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Having something with boost that can artificially alter its output(variable geometry), means that engine can burn a ton of fuel under load. IF you drive like an old lady(and I do), accelerating slowly with something like a 3.55 can burn a pile of fuel. Everything is subjective, and if you look at the Ecoboost from Ford which has a super-low power curve, it burns a ton of fuel accelerating so some get better mileage in a 3.73 than a 3.55. HOWEVER, looking at the Hemi, the power band is wider so it burns more fuel based on its RPMs (which are lower on the highway with a 3.55 than a 3.92).

BUT it's all about driving style, and although its expensive, final drive can be changed in a day.
 

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Motor Trend did some pretty good mileage testing using their new process and the highway mileage was I believe 1 MPG better with 3.55 vs 3.92, but city mileage was 1 MPG better with 3.92. Depending on your overall balance of driving the 3.92 may be a better choice given the higher tow rating and probable better performance when towing. I'm pretty sure the models MT tested were Crewcab,4x4.
 

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Discussion Starter #8
Good comments Jim and Ego,
If I end up buying a ram, I'll probably get the 3.92 gears, and 20" wheels/tires, that works out to about a 373, compared to the 17s. The driving style is key... .For most highway, the 355 would work..... but for city, Ego is right, you can dump fuel into the motor just to get it going with taller gears, where the 3.92 will operate more efficiently.

We will take trips with the truck, but not more than 3-4 a year... the rest of the time it will drive on roads near our home in Western Pennsylvania, so 3.92s are probably where I am headed.

Marsman.
 

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I think I have been convinced to go with the 3.92. Not right away but I will most likely go up one size in tires and the difference in MPG does not seem to be significant
 
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3.92 is the way to go unless you run down the highway for all your miles. Even then, if that highway involved rush hour traffic, the 3.92 might still be better
 

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The torquey diesel and 8-speed tranny are optimized for 3.55. The 3.92 is intended for heavy towing and will hurt fuel economy as well as shifting under normal conditions. At least that is what two experts told me when I asked which ratio to use in a fully-loaded vehicle. They could be wrong but I'm going with 3.55.
 

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It's 1 MPG difference according to Motor Trend. I'll take that all day long considering I don't ever hit full highway speed. 8 speeds AND variable engine/tranny management profiles. The tranny overcomes the final drive through gearing until you're in your final gear. If you care about the 1 mpg in final drive then have at er. Larger rims and tires or using all the seats while hauling anything will benefit from the 3.92
 

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OOH, and there's a huge truck/tractor pull near my house and I want to be the first diesel half class winner. 3.92 for that extra grrrrr
 

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It's 1 MPG difference according to Motor Trend. I'll take that all day long considering I don't ever hit full highway speed. 8 speeds AND variable engine/tranny management profiles. The tranny overcomes the final drive through gearing until you're in your final gear. If you care about the 1 mpg in final drive then have at er. Larger rims and tires or using all the seats while hauling anything will benefit from the 3.92
If you rarely get into top gear I can understand how lower gears would be efficient.
I live in a small city where most of my driving is long distance on highways, so high speed efficiency is important to me.
I will use stock LT 17" tires.
The 3.55 should be fine with max payload; towing is where the 3.92 would be of benefit.
WADR to Motor Trend, a singular test on a given truck means less to me than the advice of people who drive and work on Rams for a living.
 
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