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Guest Message by DevFuse

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Need Help, Axle Ratios and Limited Slip Axle


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19 replies to this topic

#1 OFFLINE   HockeyDoc

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Posted 12 April 2010 - 10:23 AM

I'm getting ready to buy a 2011 F250 Super Duty. I plan on getting the new diesel too.

I am confused about what to choose when it comes to axle ratios. Why would I want a 3.31 vs a 3.55 ratio. Also why should I get the electronic locking limited slip axle vs the non limited slip axle.

I would appreciate an explanation about these things.

I pull a boat on weekends but otherwise no major towing.

Thanks







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#2 OFFLINE   akirby

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Posted 12 April 2010 - 11:07 AM

I'm getting ready to buy a 2011 F250 Super Duty. I plan on getting the new diesel too.

I am confused about what to choose when it comes to axle ratios. Why would I want a 3.31 vs a 3.55 ratio. Also why should I get the electronic locking limited slip axle vs the non limited slip axle.

I would appreciate an explanation about these things.

I pull a boat on weekends but otherwise no major towing.

Thanks


Lower axle ratios (numerically) give you better mpg because the engine turns more slowly at the same speed. Higher ratios give you more torque (think of a 3.31 as 5th gear in a transmission versus 3.55 being 4th gear or 3.73 being 3rd gear - the difference is not that drastic but it's the same principle). For light or occasional towing it probably won't make a difference, but if you're pulling a boat I would get the 3.55 or higher just to be safe.

Limited slip allows both wheels on the same axle to turn even if one has no traction. Without limited slip, if one wheel is spinning freely in mud, ice, etc. then the other wheel won't move. Essentially, a 4wd without limited slip or lockers on either axle is really only 2wd - one front and one rear. I strongly suggest getting some type of limited slip or locker functionality.

#3 OFFLINE   rcs2419

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Posted 12 April 2010 - 01:32 PM

Go for the 3.31 you will get better mpg and if need be while towing the 2011 trucks let you lockout any gear you dont want the transmission to use IE if you lock out the top gear you will have the lower gear ratio you need for towing.

Edited by rcs2419, 12 April 2010 - 01:34 PM.


#4 OFFLINE   StevenCaylor

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Posted 12 April 2010 - 01:36 PM

Lower axle ratios (numerically) give you better mpg because the engine turns more slowly at the same speed. Higher ratios give you more torque (think of a 3.31 as 5th gear in a transmission versus 3.55 being 4th gear or 3.73 being 3rd gear - the difference is not that drastic but it's the same principle). For light or occasional towing it probably won't make a difference, but if you're pulling a boat I would get the 3.55 or higher just to be safe.

Limited slip allows both wheels on the same axle to turn even if one has no traction. Without limited slip, if one wheel is spinning freely in mud, ice, etc. then the other wheel won't move. Essentially, a 4wd without limited slip or lockers on either axle is really only 2wd - one front and one rear. I strongly suggest getting some type of limited slip or locker functionality.


Don't all new Ford trucks have traction control as standard equipment? IIRC, TC handles side to side torque application for both front and rear axles. Having a lsd isn't as important as it used to be.

#5 OFFLINE   akirby

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Posted 12 April 2010 - 03:03 PM

Don't all new Ford trucks have traction control as standard equipment? IIRC, TC handles side to side torque application for both front and rear axles. Having a lsd isn't as important as it used to be.


Au contraire. Traction control works great to keep the wheels from spinning, but it does this by braking the spinning wheel and if necessary, cutting torque altogether. Try going up a slippery hill with TC - it won't make it because the braking and torque cutout won't let you keep your momentum. In fact that's why they allow you to turn off TC - especially recommended in loose snow.

In a truck you'd want to have traction with both wheels without cutting engine power and for that you need limited slip or a locker.

#6 OFFLINE   StevenCaylor

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Posted 12 April 2010 - 06:56 PM

Au contraire. Traction control works great to keep the wheels from spinning, but it does this by braking the spinning wheel and if necessary, cutting torque altogether. Try going up a slippery hill with TC - it won't make it because the braking and torque cutout won't let you keep your momentum. In fact that's why they allow you to turn off TC - especially recommended in loose snow.

In a truck you'd want to have traction with both wheels without cutting engine power and for that you need limited slip or a locker.


Ah, understood. Snow isn't something I've had to deal with. The last time we had snow all over was January of 1949. I have TC on my Mustang. It works great for the occasional rainy road.

#7 OFFLINE   Miles2Go

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Posted 29 May 2011 - 08:30 PM

Along this line, 2011 Ford F350 offers the
3.31 Electronic Locking Axle Ratio for $390
and the
3.73 Limited Slip Axle Ratio for $2190

What is the difference between Electronic Locking and Limited Slip? (Besides $1800)
Is one fundamentally more reliable?

Thanks in Advance for any information you may provide.

#8 OFFLINE   theoldwizard

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Posted 30 May 2011 - 04:18 AM

Along this line, 2011 Ford F350 offers the
3.31 Electronic Locking Axle Ratio for $390
and the
3.73 Limited Slip Axle Ratio for $2190

What is the difference between Electronic Locking and Limited Slip? (Besides $1800)
Is one fundamentally more reliable?

Limited Slip has been around (in one of multiple forms) for many years so it is well proven technology. It is (typically) all mechanical.

Electronic LSD is relatively new and does require an electronic controller, so at least in theory it may be less durable. I don't know about the Ford system, but some of them have an ON button in the cab, so that you can just leave the axle locked for extended periods of time (gravel/dirt roads). This overrides the controller which turns it on "as required".

#9 OFFLINE   rottekatz

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Posted 01 June 2011 - 12:26 AM

The locking diff, locks the rear wheels so that they turn together and one does not spin, helps at times when you need traction.
Taller gears, with a lower ratio will increase your towing capacity. Figure the max gross vehicle weight rating for what you plan on towing to be 80% of your vehicle's capacity. the 3.73 is lower geared than the 3.31 and better suited for towing. You won't notice much difference between the two in mpg.
[size="1"]
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#10 OFFLINE   rottekatz

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Posted 01 June 2011 - 10:12 PM

A better source of the information you seek than the yahoos on here would be to google or check wikipedia for limited slip differential; locking differential; and open differential. Then check the Ford site for tow capacity and gear ratio. Then buy a GMC or Ram.
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2011 Cherry Red, Ram 1500, Big Horn, Hemi, On Order
Previous
2011 F250, 6.2, 43 days in dealer shop for repair, possibly last Ford,13 weeks for buy back, lemon law
2008 Nissan Altima
2006.5 Kia Optima also trade for 2011 junk
2006 Ford Focus ZX3
2xxx Buick LaSabre Traded for Kia
2xxx Honda CRV, killer seats, traded for Buick
2001 F250 traded for 2011 6.2L POS junk
2xxx Mercury Grand Marquis
1997 Ford Ranger, traded for 2001 F250
1992 Fiat Ducato Panorama, was stationed in Italy
1995 Ford Crown Vic
1992 Honda Civic, wife wanted bigger car, traded for Crown Vic
1989 Ford Aerostar, , traded for Crown Vic
1985 Mercury Marquis, Sold to German, in Germany
1965 Chevy 1/2T
1976 Datsun P/U
1980 Toyota Corolla, traded for Marquis, my last Toyota
1971 Chevy Malibu (4 door)
1976 Toyota Corona
1974 AMC Hornet Sportabout, best handling car, our 1st new car
1968 Honda mini 360, only in Japan
1969 Volkswagon, OUR 1st car
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#11 OFFLINE   fordmantpw

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Posted 01 June 2011 - 10:28 PM

Along this line, 2011 Ford F350 offers the
3.31 Electronic Locking Axle Ratio for $390
and the
3.73 Limited Slip Axle Ratio for $2190

What is the difference between Electronic Locking and Limited Slip? (Besides $1800)
Is one fundamentally more reliable?

Thanks in Advance for any information you may provide.


The reason for the cost difference is because the 3.73 comes only with 20" wheels, so much of that difference is the cost of the larger wheels/tires.

If you don't tow heavily, go with the 3.31 for better fuel economy.

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#12 OFFLINE   jamesgang

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Posted 15 June 2011 - 11:56 AM

I have 3.73's with 17" wheels & elec. locking rear, but the elec. locker will unlock above 25 mph.

#13 OFFLINE   MTSuperCrew

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Posted 28 September 2011 - 09:58 AM

So is the electronic locking diff a regular limited slip during the time it isn't locked up? Nobody has been able to answer this at the dealership.

#14 OFFLINE   akirby

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Posted 28 September 2011 - 10:44 AM

So is the electronic locking diff a regular limited slip during the time it isn't locked up? Nobody has been able to answer this at the dealership.


It should be an open diff when it's not locked. The electronic locking mechanism takes the place of the mechanical limited slip locking mechanism.

(this is my assumption - could be totally wrong).

#15 OFFLINE   fordmantpw

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Posted 28 September 2011 - 10:45 AM

I'm 99.9% certain that akirby is correct.

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#16 OFFLINE   MTSuperCrew

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Posted 28 September 2011 - 12:12 PM

I hope that isn't the case, having a limited slip while driving in the snow up mountain passes sure helps and we're usually rolling about 40mph so I couldn't engage the locker at that speed. Might have to yank that electronic locker out and replace it with an old school limited slip.

#17 OFFLINE   fordmantpw

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Posted 28 September 2011 - 12:17 PM

I hope that isn't the case, having a limited slip while driving in the snow up mountain passes sure helps and we're usually rolling about 40mph so I couldn't engage the locker at that speed. Might have to yank that electronic locker out and replace it with an old school limited slip.


Ummm, use 4x4? If the going gets so tough that 4x4 (with a non-LS rearend) won't pull you through, you shouldn't be going 40 MPH.

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'03 Ford Escape XLT
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'96 Chevrolet Cavalier (wife's - she brought it into the marriage)
'86 Ford Bronco II XLT 4x4 5 Speed 2.9L - what a dog, and it cracked heads like no tomorrow. 150k+ on 4 engines


#18 OFFLINE   MTSuperCrew

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Posted 28 September 2011 - 12:25 PM

With a gooseneck enclosed snowmobile trailer, we're loading the rear axle enough that we usually don't need 4x4 as long as both rear tires are doing the work. Also drives better and gets better mileage by staying out of 4x4. In the later parts of winter, the mountain pass will end up with dry spots and icy corners so with a limited slip we don't have to engage 4x4 on every corner and then turn it off til the next corner.

The limited slip worked so well that I'm nervous going back to a one-wheel wonder. The last open diff I had, I got stuck on a 12" x 12" patch of ice in the Mc D's drive thru. Had to get out and turn the hubs in so I could move. LOL

#19 OFFLINE   fordmantpw

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Posted 28 September 2011 - 12:37 PM

With a gooseneck enclosed snowmobile trailer, we're loading the rear axle enough that we usually don't need 4x4 as long as both rear tires are doing the work. Also drives better and gets better mileage by staying out of 4x4. In the later parts of winter, the mountain pass will end up with dry spots and icy corners so with a limited slip we don't have to engage 4x4 on every corner and then turn it off til the next corner.

The limited slip worked so well that I'm nervous going back to a one-wheel wonder. The last open diff I had, I got stuck on a 12" x 12" patch of ice in the Mc D's drive thru. Had to get out and turn the hubs in so I could move. LOL


A locker at highway speeds can be pretty dangerous, hence the reason it unlocks at speed. You only have a "one-wheel wonder" at speed since you have a e-locker. Plus, you don't have to lock hubs any more! :)

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Current Vehicles
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'09 Ford Flex Limited
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'05 Ford F150 XLT SuperCrew 4x4 5.4 - sold @53k, not a single problem
'03 Ford Escape XLT
(wife's) Traded at 75k for '09 Flex
'99 Ford F150 XLT Regular Cab 4x4 5.4 - Sold @136k, then totaled by the new owner on the way to its new home :(
'96 Chevrolet Cavalier (wife's - she brought it into the marriage)
'86 Ford Bronco II XLT 4x4 5 Speed 2.9L - what a dog, and it cracked heads like no tomorrow. 150k+ on 4 engines


#20 OFFLINE   akirby

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Posted 28 September 2011 - 01:38 PM

According to this F150 online forum topic it is open and can only be locked below 25 mph - and then only off road where the tires can slip.

The difference in the newer trucks is that you have electronic Traction Control that will function similarly to a LSD in that it will send power from one wheel to the other if one starts to slip. So you won't get stuck when one wheel breaks loose. This should be sufficient for high speed driving in snow or rain even with the open diff.