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Old 07-15-2010, 09:52 AM
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Deciding between Axle ratios

I do believe I'll be pulling the trigger very soon to buy a 2011 diesel, but I'm still unsure about which axle ratio would be best for me ; I'm hoping to hear some opinion / recommendations.

I know I do not need a low ratio and I am undecided between a 3.31 and a 3.55. I tow a rather light 3600 # (dry) camper trailer during holidays and during hunting season. Aside from that I'd mostly be running without a load. I am drawn to the 3.31 as I feel it would be the best overall for my case ; I'm sure I'd be in 5th while towing that trailer, and the RPM at highway speeds would likely be very similar to my current 7.3 (3.73). Without a load I know the RPM would be very nice and low, but I wonder how often it would need to shift to 5th going up a hill.

I know that the 3.55 would be pretty much at peak torque at highway speeds, so I am sure that the 3.55 would seldom downshift going up hills unloaded. Not sure if it would need to downshift much towing my trailer in 6th ; I suspect it could maintain 6th pretty easily.

Finally I wonder if the 3.31 would have more regenerations than a 3.55 since I would assume more soot would develop since it would largely be running outside its peak power range. And with more generations, I wonder if 3.31 would really have much of an edge on fuel economy.

I would appreciate any opinions on the topic.

Thanks,

Rob
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Old 07-15-2010, 10:35 AM
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I have the 3.55 locker and the 20 inch wheel package. I tow up to about 8k lbs. And that is very easy to do.

This diesel has so much torque, on flat ground driving empty vs. towing the trailer is almost the same!! In hills you will notice the load but nothing like you would with gasoline power. I live in the desert so our land lays on a slight grade almost anywhere you go. At the edge of the valleys you climb some steep grades to get over a mountain and then back into another valley area.

I grew up in the corn belt and I remember the rolling hills too. I believe in rolling hills you would be shifting between 5th and 6th quite a bit. You may even want to lock 6th out in those conditions when towing.

I think the 3.31 ratio might suit you best since you pull a light trailer - - - sometimes. Most of your driving is empty too so that makes it an easy decision. The Scorpion has the guts to make you proud those few times you will pull the trailer.

I got the 3.55 because the truck had them when I picked it out on the lot. If I had ordered my truck, I would have been doing what you are right now, asking questions.

One more thing is fuel milage. If you read the milage posts it looks like the 3.31 does a little better than the 3.55 on empty trucks, which you will be most of the time.

Not trying to confuse you and I do understand - this is a very big decision and you are doing your homework. Good for you and good luck with your new truck!!
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Old 07-15-2010, 11:37 AM
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Don't you have to get the 3.55 if you select the 20" wheel option?
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Old 07-15-2010, 12:51 PM
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Are you going to keep the stock wheel/tire setup? If you are going to go taller than stock at some point I would opt for the 3.55 ratio.
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Old 07-15-2010, 01:24 PM
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Since you are in the planning stages, another thing to consider now would be IF you would be moving up to a bigger trailer later. If that's a possibility, you might want on lean toward the 3.55s.
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Old 07-15-2010, 01:50 PM
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Good points

I am very unlikely to alter the tire size - never had the desire to do so with my 1997 7.3.

As to trailer, that can certainly vary, but I think I'd not likely get a bigger trailer ; I'm pretty happy with the one I have and it fits the needs of my small family.

Question on towing with either 3.31 or 3.55. When towing such a light trailer (3600# dry), would you expect the 3.31 to be working no higher than 5th gear? Would the 3.55 be able to sustain 6th?

Do most people tow outside of the top overdrive gear?
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Old 07-15-2010, 03:16 PM
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I have 3:31's and a taller tire on stock 17" rims(1.2" taller). I pulled about 25K and it pulled alot easier than my 5.9 with a 65hp chip and 3:73 gears. I say get the 3:31's unless you are putting monster tires on and want to pull 30K.
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Old 09-12-2010, 07:05 PM
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I went through the same question last week at the dealership. I went with the 3.55's, only because Last year I bought a 6.7 cummins with 3.42 gears and when towing my 5th wheel, about 16k, the mileage was horrible. I didnt expect it to be such a difference and tried to go for better fuel economy, which it had when not loaded. I pulled the same trailer with my company cummins, 6.7 with I want to say 3.73's and it was about 3mpg more economical. I pick up my 2011 f-350 tuesday, and friday I am pulling the trailer from waco tx to portland oregon. I will let you know how it goes.
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Old 09-12-2010, 07:56 PM
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Quote:
Originally Posted by Windturbineguy View Post
I went through the same question last week at the dealership. I went with the 3.55's, only because Last year I bought a 6.7 cummins with 3.42 gears and when towing my 5th wheel, about 16k, the mileage was horrible. I didnt expect it to be such a difference and tried to go for better fuel economy, which it had when not loaded. I pulled the same trailer with my company cummins, 6.7 with I want to say 3.73's and it was about 3mpg more economical. I pick up my 2011 f-350 tuesday, and friday I am pulling the trailer from waco tx to portland oregon. I will let you know how it goes.
Please do let us know how you come out on your mileage pulling the 5er.
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Old 09-16-2010, 01:00 PM
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After reading and see what you would be using the truck for? The 3.31 will serve you very well and probably be over kill.
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