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Old 09-30-2011, 05:54 AM
MKEBoost MKEBoost is offline
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3.55 Rear Axle?

So I've seen a few posts about rear axles and ratio's etc but I don't see too much mention of the 3.55 ratio with the EcoBoost motor in the F150 4by. Yet when I configure a F150 SCrew 4by with Eco, it automatically defaults/adds the option for 3.55 LS ratio. Also, most if not all F150's I've seen on the lots around here with the Eco have the same 3.55. Is there something I'm missing here...why are most people posting that they have something different? I hope the 3.55 will be good, 'cause my truck is on the way from the factory with it in it.
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Old 09-30-2011, 06:25 AM
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A lot of folks are special ordering their new F-150's with 3.73:1 gear ratio. It will improve towing and help for those that put larger tires on their trucks. The 3.55:1 setup is an average mid-range ratio good for highway driving and is probably considered standard equipment on a lot of models. Don't expect to get the best fuel mileage with a 3.73:1 but if you look at a lot of picture of some of these truck you would think there isn't much consideration except trying to make the truck look good.
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Old 09-30-2011, 06:54 AM
Ralph Ralph is offline
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I'm trying to figure out the same thing! I want to order a new 2012 in the next few days and trying to figure out which axle is the best choice. I thought standard was 3.31 and a optional 3.55 or 3.73 was available?
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Old 09-30-2011, 07:17 AM
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Originally Posted by Ralph View Post
I'm trying to figure out the same thing! I want to order a new 2012 in the next few days and trying to figure out which axle is the best choice. I thought standard was 3.31 and a optional 3.55 or 3.73 was available?
On page 42 and 43 of the 2011 F150 brochure it lists all of the models available and which axle ratio is available. The axle ratio's go from 3.15 to 4.10.

These are the axle ratio's listed under, CONVENTIONAL TOWING - MAXIMUM LOADED TRAILER WEIGHT RATINGS (LBS)

3.5 Ecoboost
3.15
3.31
3.55
3.73
4.10

3.7 V6
3.55
3.73

5.0 V8
3.31
3.55
3.73

6.2 V8
3.73
4.10

I'm not sure if this means these are the only axle ratio's available for the respective engine or not.
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Old 09-30-2011, 08:14 AM
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IMO, unless you will be towing >5,000 lbs much of the time or for long distances occasionally, the 3.55's will be just fine, even with 33's.

You have to keep in mind that the 6-speed has some very low gears (1: 4.17, 2: 2.34, 3: 1.52, 4: 1.14, 5: 0.87, 6: 0.69) and you have the ability to lock out O/D so you should be able to keep the engine at or near peak torque for most of your driving needs.
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Old 09-30-2011, 12:00 PM
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I mentioned this the other day in another thread, and have to echo Augster's thoughts - unless you're towing heavy, 3.73s with the Eco is overkill in my opinion. I went with 3.73s and would have been fine with 3.55s.
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Old 09-30-2011, 01:05 PM
simprog1700 simprog1700 is offline
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I have a 3.55 in mine and am more than pleased with it.
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Old 09-30-2011, 03:12 PM
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I tow an 8000 lb trailer with 3.55's and absolutely love it. No regrets here.
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Old 09-30-2011, 03:38 PM
Truckpilot1329 Truckpilot1329 is offline
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If you're driving down the road in your truck equipped with 3.31 gears, going, say about 65 and let's say that tach is reading 1600 rpm.
If you were driving with 3.55 gears, you'd be taching 1700 rpm.
With 3.73 gears you'd be taching 1800 rpm. It's a linear function.

4.10 gears would put you at 2000 rpm. Now we're taking some additional fuel burn.

It would be nice to test drive various geared vehicles and see what the difference is, but most of us don't have access to a bunch of test trucks.

My experience is that getting high speed gears, like 3.31s for better milage in a mountain environment can backfire, as the vehicle has to downshift sooner going uphill, whereas it might make the grade in a higher gear. The fuel burn on a downshift is very pronounced. The engine has to make the same HP to get up the grade at a constant vehicle speed, but is more fuel efficient at the lowest engine rpm.

Another point of interest is that some F-150 4x4 models with the 18 inches wheels which means 275x65x18 tires.
The optional 20 inches wheels come with tires that are 275x55x20.

What interesting is that both these tires sizes are virtually identical in diameter; 32 inches, for whatever that is worth.
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Old 09-30-2011, 05:24 PM
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[quote=Truckpilot1329;10869997]If you're driving down the road in your truck equipped with 3.31 gears, going, say about 65 and let's say that tach is reading 1600 rpm.
If you were driving with 3.55 gears, you'd be taching 1700 rpm.
With 3.73 gears you'd be taching 1800 rpm. It's a linear function.

4.10 gears would put you at 2000 rpm. Now we're taking some additional fuel burn.

It would be nice to test drive various geared vehicles and see what the difference is, but most of us don't have access to a bunch of test trucks.

My experience is that getting high speed gears, like 3.31s for better milage in a mountain environment can backfire, as the vehicle has to downshift sooner going uphill, whereas it might make the grade in a higher gear. The fuel burn on a downshift is very pronounced. The engine has to make the same HP to get up the grade at a constant vehicle speed, but is more fuel efficient at the lowest engine rpm.

Another point of interest is that some F-150 4x4 models with the 18 inches wheels which means 275x65x18 tires.
The optional 20 inches wheels come with tires that are 275x55x20.

What interesting is that both these tires sizes are virtually identical in diameter; 32 inches, for whatever that is worth.[/QUOTE]

That's worth a lot as there are tons of tire related questions on here. Many folks don't realize that little tidbit about factory 18's and 20's.

For the ecoboost engine, I would consider the 3.55 axles to be optimal unless one is towing 11k every day.
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Old 09-30-2011, 07:57 PM
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My boss's King Ranch ECO has 3.55's, and that truck has plenty of power. 373's are probably better left for towing and hauling.
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Old 10-01-2011, 03:58 AM
MKEBoost MKEBoost is offline
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Wow..great information from everyone-thanks. My truck was ordered with the 18" factory wheels and as mentioned, the 3.55 limited slip rear which sounds like it will be fine. I'm not planning on changing tire size or towing large loads (or any load for that matter).
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Old 10-01-2011, 05:29 AM
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Quote:
Originally Posted by MKEBoost View Post
Wow..great information from everyone-thanks. My truck was ordered with the 18" factory wheels and as mentioned, the 3.55 limited slip rear which sounds like it will be fine. I'm not planning on changing tire size or towing large loads (or any load for that matter).
But the good thing about the ecoboost engine with any gear set is that you can tow. I believe the 3.15 axle gets you a 7500 lb tow rating. That speaks volumes for the engine / transmission duo.

With the 3.55, you'll be more than well equipped to handle most any task that an F-150 is rated for.
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Old 10-01-2011, 06:04 AM
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Originally Posted by tseekins View Post
But the good thing about the ecoboost engine with any gear set is that you can tow. I believe the 3.15 axle gets you a 7500 lb tow rating. That speaks volumes for the engine / transmission duo.

With the 3.55, you'll be more than well equipped to handle most any task that an F-150 is rated for.
I wonder what kind of fuel mileage one could get with an EB and 3.15:1 setup.
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Old 10-01-2011, 08:36 AM
LCPullman LCPullman is offline
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I have the ecoboost with the 3.55 axle. It tows 6000lbs very well, and when its empty I can still get about 23mpg on the highway.
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Old 10-01-2011, 08:36 AM
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