3.5 Ecoboost with 3.31 or 3.55 rear axle?

DDRAHT

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I'm shopping pickups and looking at 2017 or newer F150s with 3.5 Ecoboost and 10 speed transmission. Any real world experiences (or opinions) about 3.31 vs 3.55 rear axle? I tow some on the weekends (18.5' fiberglass boat, 16' enclosed trailer, 16' dump trailer) otherwise mostly highway and a little city driving.

Reading a bunch of Ford forums, some guys say definitely go 3.55 while other guys say 3.31 will do just fine.

I was leaning toward finding a truck with 3.55 and then I found a 3.31 that looks to be decent. I'm definitely not in a "must buy" situation, but I wouldn't want to let a good deal get away. :D

Thoughts?
 


eyexer

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I just bought a 2011 with 3.55. Haven’t picked it up yet. Its out in Ohio. From Chicago east there are some smokin deals on F150’s that are being brought down out of Canada. Look on auto trader. If you don’t mind flying somewhere and driving it back. I saved at least 4K buying it over there.
 

Captain Ahab

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I would imagine it would find one of the 10 speeds to tow well with either. Probably only a slight factor right off the dig.
 

BrokenBackJack

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I was told many years ago if doing any towing always go with the lower geared rear-end. For you this would be the 3.55.
Just my 2 cents.
 

Sparky714

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I've got a 2019 3.5 eco with the 331 rear end and 10 speed transmission. I pull a 5000lb trailer at 65mph and hardly know it's back there. Set the cruise at 65 and it never drops to less than 63mph and down to 7th gear on the steepest hill.
 


Snapper

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FYI: I just put my 2018 F-150 Limited with a 3.5 Ecoboost on Bismanonline. Photos will be added today. Shadow black.
 

SDMF

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Dump trailer is the only reason I'd choose 3.55's.
 

shorthairsrus

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I have a 3.55 in mine - the thing never down shifts towing and going up a hill ------all that happens is the turbos kick in and the fuel goes slurrpppp.
 

Sub_Elect

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3.55 rear end if you are going to pull anything. I wouldn't consider a 3.31 unless it will only get groceries!
 

Davey Crockett

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Is there even a noticeable difference in MPG between the two when running down the road empty ?
 


Up Y'oars

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I sold my F150 back in August and it had the 3.31 rear end. MANY of the comments on the Ford F150 forum showed that the 3.55 rear end was faring better than the 3.31. I didn't like the rear end and while it pulled my fiberglass boat just fine it had zero advantage with gas mileage. I'd think the mpg's might improve some while pulling a trailer with the lower geared ratios as long as you're driving conditions are normal (wind, snow, heavy foot, etc.).
 
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gone_fishing

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With these 10 speed autos, I don't think having a lowest rear axle ratio available is as big of deal as it used to be (for towing purposes). With 10 speeds to work with, the transmission should be able to find a ratio that gives a good balance of power and economy. The ratio spread is a lot larger too (between 1st and final). Not like the old 3 or 4 speed days where you get maybe 2 non-overdrive gears to choose from for cruising (since most don't tow in OD..at least nothing heavy). 3rd gear might not be enough but 2nd gear shot the RPM's through the roof! Since you didn't have anything in between, it helped to have a lower axle ratio to help increase your RPM's and get torque to the ground (making 3rd gear more manageable). Also, these turbo gas engines have almost diesel like torque curves so they don't have to rev as much to make power compared to a NA gas engine. This gives you a wider power band to work with.

I don't know about the Fords but on some GM 1/2 ton models, going with the lower rear axle increases towing capacity (by a couple thousand pounds even). If it's the same way in the Fords, this might be worth looking into depending on how much you plan to tow. I doubt there is much difference in economy between these two choices anyways since they are so close together mathematically.
 

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