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Discussion Starter · #1 ·
First post on forum, been reading for a while and really appreciate all the information out there.

I am looking to purchase a new truck and my biggest sticking point is getting either the 355 or 392. I do a strong mix of highway and in town driving, I also plan on getting the 4x4 Crew Cab Laramie. I live in a hilly area but not overly hilly, but will be moving to CA where it is rather hilly with mountains etc.

I don't plan in lifting or getting bigger tires, nor will I be doing much towing. What is the forums suggestion in the 392 or 355 based on this summary?
 

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I'd go with 3.55. This is all about engine rpms. The transmission is entirely capable of setting your engine rpms under all but the most extreme situations like going up the most steep pass in the Rockies. The 3.92 is speced with heavier towing capacity because "capacity" is all about those extremes. I'm pretty much at the cargo capacity and towing limit for my 3.55 and I've no worries because there isn't a real mountain within 1000 miles of me.

Taller diff for better freeway mpg. Shorter 3.92 diff for towing a house up and over the Rockies in first gear.
 

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If you were buying a Hemi, then going to 3.92 might make sense. But as Ranger said - 3.55 puts you in the right range of RPM's for this Engine - depending on what you are towing of course. For me - it will be a boat (probably about 3000 pounds) or a pop tent trailer. From my research the engine won't even notice I am pulling anything with a 3.55 backend.
 

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Welcome.

If you are not towing HEAVY I would only think 3.55. Now if you were towing ordinary loads to say 6,000 lbs. The 3.55 is fine. More weight? Depends on the terrain. The transmission will allow the 3.55's to pull fine. If you do heavy towing all the time and it's hilly, then go to the higher gear ratio.

From what you said the decision is a "no brainer".
 

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Yep... 3.55's all the way. This engine can lug down, and still make good torque. Also... since first gear is 4.7:1 (and second gear is lower than 1'st on the old 6 speed) you don't need the 3.92's to get rolling if you are towing under 6K Lbs. You will take a hit on your MPG's when not towing with the 3.92's.
 

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VERY sound advice by earlier posters. 1st gear is very steep!
 

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I had the same back and forth. If you are not towing any significant, no point in 3.92. I will pull a 6000lb - 6500 lb travel trailer. I think the 3.55's will be just fine, but will probably go with 3.92 just for the hell of it.

A member on here put it in a way that "clicked" with how my mind works. Look at it like this. Assume when I am not towing (majority of time) I would get 27 highway with 3.55 and 25 highway with 3.92. I don't know what the true mpg loss is, but just guessing. After 175,000 miles at $4 a gallon, that is about $2000 difference in total fuel. That will take me 10 years so $200 a year. For me, that $200 a year doesn't get me that worked up and I can have a little more towing capability just in case. Ask yourself if that $200 or so a year "bothers" you.

First post, been lurking for a while. Good information on here. I get my ED this winter!

Edit: I think the posts by Ranger and Captain are spot on if you don't like the "does the cost matter to me" way of thinking.
 

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Sounds like 3.55s will work for you, (stock tires, light to no towing) if those turn out to be too high, you could always put 17” wheels on it. However if you want performance for hills, and aren’t always rolling 80 down the highway unloaded go for the 3.92s!

I’m not convinced the 3.92s will really hurt fuel economy all that much, I think MPGs are more dependent on the way the truck is driven and top speeds. A diesel doesn’t need to maintain as tight of air/fuel mixture band as a gasser. Fuel economy is more related to engine load.

I think towing capacity (3.55 vs 3.92) is probably related to the capacity of something like the differential. For a given output required torque the 3.55 will require more input torque than 3.92s, hence increasing the differential bearing loads. That additional torque needs to come through the transmission and be generated by the engine via. more fuel.

It is easy to design a system like this for maximum efficiently when the loads are known and don’t fluctuate, however RAM doesn’t know how each of us will use our trucks.

Ideally all components will have close structural limits, since overly built components usually add unnecessary weight.
 

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355's all the way, for what you described
 

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Mine is Laramie, CC, 4x4 with 3.92. I'm happy with that ratio. I live in hilly/mountainous area and am averaging 22-24 mpg with local driving. I do plan on towing, so that was part of the decision process. I don't think the 3.55 will provide much better mileage in hilly terrain.
 

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3.55 and 3.92 are not going to make much of a difference in fuel economy. Thats why its only $100 difference. The tow rating is by almost 2000lbs of a difference. To me get the 3.92.

At 95km/h the engine revs at 1500rpm. At 110km/h the engine revs at 1800 rpm. At 120km/h the engine revs at 2000 rpm.

Iam avergaing 25mpg city and hwy combined. highway strictly I get 29mpg.

3.92 are better towing gears anyway. I am not sure why everyone is saying 355's are better. Mind you I am not an expert in this matter. They use 355 gearing in cars. Heavy duty trucks normally use 4.10's and up.
 

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Discussion Starter · #12 ·
I really appreciate everyone's input here. Site has been a great resource before purchasing. Unfortunately I'm still a bit torn, lol. If I go with the 20 inch wheels it sounds like the 392 still might be the best bet to go with. The mileage might suffer some for everyday driving but the road trips through hills/mountains for longer distances may actually be better. If I reading everyone's comments accurately?
 

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I am not an expert and I, like you, have been going back and forth. There are good arguments for both. That is why, at the end of the day, I figured up a yearly cost estimate and let that be my tolerance for possibly "wasting" that much on something that may not be necessary be my deciding factor
 

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If you can test drive one to help you decide....
 

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what is not being discussed/pointed out ... your resell value with 3.92 will outweigh any minimal MPG difference in the 3.55 .. for < $40 there's almost no reason NOT to go 3.92 ... talk to your local Dodge dealer (heck talk to a few of them), ask them what people are looking for when buying used trucks. for those with 3.55, my comment is not an attempt to say your reasoning is flawed or you made a mistake. to me it was after talking to a few dodge dealers, they all had the same comment - go with 3.92 gears, it will be worth more later than the minimal MPG loss. so that's what i did. assuming my truck ever gets here. and it was built as ordered. ;)
 

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Diesel fan, lot of good posts here, and it comes down to preference..... really, and also another question;
want vs need, do you "need" a 392? answer in many cases is "no" a 355 will work.

Other considerations; where do you live, what terrain will the truck be used in, 90% of the time, will it be every day commuting? out of 1000 miles, how many of those 1000 will be towing? percentage pure highway miles vs commuting, vs every day driving....

With the 8 speed, and .667 final drive, either will work. I will probably opt for 392s.

Also if you calculate the ratios with tire diameter (17s are 31.6" and 20s are 33") the 355 with 17s vs with 20s, or 392s with 20" vs with 17s both get you closer to a 373 with the opposite tire size., so keep wheel/ tire size as part of your equation.

I like the highway argument if you are retired, take a lot of trips, or live in a city or state where you are on highway a lot of the time, and area is flat, but really how many owners are in that situation (Capt in Florida). And with 392s even for highway, if you slow 4-6 mph on highway, the fuel economy should be no different than the 355. Both are good, no bad choices here, more personal preference.

In our next town, Zelienople, Pennsylvania, the Fire Dept raffles a Mustang every year, well boys this year it's a Laramie Longhorn, Diesel, 355, 2 tone white and gold... will have to drop a buck on a few tickets (with no chance of willing)..... Will post a picture, truck is sharp. Marsman
 

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Also if you calculate the ratios with tire diameter (17s are 31.6" and 20s are 33") the 355 with 17s vs with 20s, or 392s with 20" vs with 17s both get you closer to a 373 with the opposite tire size., so keep wheel/ tire size as part of your equation.
That's a good point... and needs to be considered. I got the 17" wheel and tires. I like the looks of the 20's... but with all the potholes, and bad roads in PA... the taller sidewall is better. If you are towing a lot, and are getting the 20's... then I would go with the 3.92's.


In our next town, Zelienople, Pennsylvania, the Fire Dept raffles a Mustang every year, well boys this year it's a Laramie Longhorn, Diesel, 355, 2 tone white and gold... will have to drop a buck on a few tickets (with no chance of willing)..... Will post a picture, truck is sharp. Marsman
NO shit !!

I guess I'll have to wonder up there, and buy some tickets too.
 

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Buy some tickets.

I doubt resale will be helped with the 3.92. First there are few who can even figure out what the rear-end ratio is on a used truck. Most buyers will be clueless as to what that even is. Then there are many who will find out the rear-end ratio and back away. No way will a normal person want 4:10's on most pickups because of the historical problems with fuel mileage and higher rpm's.

My take is there might be a small percentage of used buyers that would want the higher rear-end ratio and might pay for that. Strongly feel it will deter more than it attracts and most will be clueless.
 

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Another long 3.55/3.92 thread??? Any new info?

Many strangers have asked about our truck and the question is always, "What mileage you getting?," never "What rear axle ratio you got?"
Performance under normal driving and hauling is optimized for 3.55, so unless there is a good reason for going lower (such as oversize tires or frequent, heavy towing) the choice is obvious.
I have driven about 2000 miles now over hills, dales and mountains; the 3.55 has ample power.
It seems to like 1600 rpm when level highway cruising and 2100 rpm up grades.
When passing the little diesel pulled to 90 mph before I realized how fast I was going.
 

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Buy some tickets.

I doubt resale will be helped with the 3.92. First there are few who can even figure out what the rear-end ratio is on a used truck. Most buyers will be clueless as to what that even is. Then there are many who will find out the rear-end ratio and back away. No way will a normal person want 4:10's on most pickups because of the historical problems with fuel mileage and higher rpm's.

My take is there might be a small percentage of used buyers that would want the higher rear-end ratio and might pay for that. Strongly feel it will deter more than it attracts and most will be clueless.
Agreed. Good friend who is a GM at a Dodge dealership. In most cases, on a 1/2 ton pickup, in no way is the deal going to be swayed by the real axle gear ratio.
 
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