F350, 3.31 vs. 3.55 rear axle ratio

[Jerry Burks]

Looking at the tow ratings for the F350 (diesel, short bed, crew cab, 4x4), it seems that there is no difference between the 3.31 and 3.55 ratios.
Obviously the 3.55 ratio may provide a little more oomph but with a planned max trailer weight of 15,000 lbs I guess it should not matter? Could the 3.31 ratio save a drop of diesel?
Thanks for any insight!

[twinboat]

No, fuel burn is determined by the power needed to move the load down the road.

A slower turning engine is still creating that needed HP.

[swavescatter]

Originally Posted by twinboat

No, fuel burn is determined by the power needed to move the load down the road.

A slower turning engine is still creating that needed HP.

Not sure I agree with that at all.

Taller gears (lower ratio) allow the truck to spin at lower rpms for any given speed. This saves fuel when cruising. The motor is always burning the same amount of fuel for a given rpm and throttle input, but lower rpms = less fuel all else considered.

I just bought a new 21 F350 longbed with the 3.55's. I'll probably run 37's eventually so I think that's a perfect ratio for my setup. However if you plan to run stock tires and want to save some fuel/extend range then 3.31's should get you great fuel mileage.

[twinboat]

Originally Posted by swavescatter

Not sure I agree with that at all.

. The motor is always burning the same amount of fuel for a given rpm and throttle input, but lower rpms = less fuel all else considered.

.

That's not true.

If fuel burn only depended on RPM, economy wouldn't go down when hauling a heavier load at the same RPM.
Add load to an engine and it needs more fuel to move it at the same speed, regardless of RPMs.

[nicky8668]

You won’t be able to use the full potential if you get the higher gear (lower number).

[JDT]

I've watched this argument play out on several forums and the thinking is the 3.55 has to be way better. No offense intended but probably many of those giving the advice have not owned or even driven an F250/350 with the new 10 speed transmission. As an owner of one with over a years worth of experience behind the wheel towing and not towing, I'm telling you the new transmission is a game changer.

I opted for the 3.31 because we do take some long trips solo and I was hopeful it might give me a little better mileage. The gearing is close enough that I didn't think there would be that much difference when I'm towing our 14,700 pound, loaded for a trip, Pinnacle. Our prior truck was a 17 F350 dually towing an Excel fifth wheel that weighed about 1500 more pounds. Maybe not a fair comparison but the new 20 has a smoother and more powerful feel than the 17 did. It gets up to speed with the Pinnacle very quickly and unless the inclines are fairly steep it holds 10th gear very well. There is absolutely no gear hunting. It just downshifts to where it needs to be when the conditions call for it as is the case for most trucks.

I would say that either ratio will work for whatever you want it to do within the trucks limitations, of course. If you will use it for nothing but towing maybe the 3.55 is the better choice but do not be afraid of the 3.31. I've posted this before but here is a picture of my trip meter for a 722 mile trip to Chicago last October to help my sister-in-law remodel her house. I had my wife, mother-in-law and a bed full of tools for the trip. I'm happy.

[swavescatter]

Originally Posted by twinboat

That's not true.

If fuel burn only depended on RPM, economy wouldn't go down when hauling a heavier load at the same RPM.
Add load to an engine and it needs more fuel to move it at the same speed, regardless of RPMs.

When I say throttle input, I mean fuel needs to the engine. More load = more throttle (electronically fly by wire, not pushing with your foot).

With your logic, identical trucks, one running 4.56 gears and 6 speed transmission, and one running 3.31 with the new 10-spd would get the same fuel mileage. We all know this isn't true.

Gear ratios matter, but as another use pointed out these gear sets are fairly close and the new transmissions take care of a lot of (relative) deficiencies.

[Dieselguy4]

Running empty the 3.31 gears with the 10 speeds .632 overdrive should get better mileage due to lower rpms at highway speeds. Towing the 3.31s or the 3.55 gears will be the same or close. Pulling a camper you will see 10 MPG +-. You need to make HP/Tq to move that trailer. The trans will up shift to keep the engine in the correct rpm no matter what gears. I have the 4.10s and 6 speed in my 17 F350. I love the combo for pulling. Unloaded fuel mileage 16 MPG. My sons 19 F350 3.55 gears 6 speed will get 20 MPG empty 10 MPG pulling. I op for 3.55s

[Elev8torguy]

355 gears 2020 f350 6.7 real world mixed driving empty. This was post fill up and then hand calculated. The computer was almost dead on if I recall correctly.

[SGhost]

Based on what I've been able to pull from the conversation, 3.31 should be fine in most conditions with the new 10 speed unless you have a heavy trailer (over 15K) and pull in very steep terrain. Unless your going to special order the truck (could take 3 months) finding one on the lot is like hitting the lotto.......

[Jerry Burks]

I did go for the 3.1 ratio. There was another one at the dealership with 3.55 but way more bling and gadgets and more expensive.

I created a spredsheet to calculate the (theoretical) motor revs at 65mph in various gears with typical 17" wheels (660 revs/mile). See attachment. Given the tight steps between gears I suppose the mpg number will not change much and I doubt the 3.31 ratio truck will ever change to 10th gear when towing at 65mph.

[JDT]

I may need to pay closer attention but I think mine shifts into 10th somewhere between 55 and 60 and holds it well. I have a 200 mile tow coming up Thursday. I'll pay closer attention to what it does, if I remember.

[tuffr2]

My thoughts, I am one that does not have a new 10 speed but here goes.

3:31 will in fact get better mpg than the 3:55. But you would need very sensitive scientific instruments to measure it as it would not amount to a hill of beans.

[runamuck]

less than 7% dif. if it wasnt a dsl you might feel a little more get up and go with the lower gears..but doubt if most folks would.