Ring & pinion upgrade

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Gunner

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I have a 1999 4x4 exped 5.4 with 9.75 and 3.31 ratio. I am wanting to change to 3.73 due to pulling a travel trailer. Do I need to change front ratio too.
 

ExplorerTom

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Yes. Certainly if you ever want to use 4wd. If you have the a4wd, yes as well.
 

1955moose

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Just curious Tom, what would happen if you mixed the two ratios? Would your front differential pull, albeit at a higher speed ratio, Or would you smoke the front differential completely?


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ExplorerTom

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If the transfercase can completely disconnect the front from the rear, then nothing should happen. One diff will be spinning faster than the other. But nothing other than that. Once you engage the transfer case, you better be on a very slippery surface or something will break due to the different ratios.
 

JoeD

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Just dont do it, small variations can be felt while moving. Which is one reason its stated not to use 4wd on dry pavement.
Catatrophic failure and death may result.

Not all ratios can be matched perfectly front and rear, depending on manufacturer.
 

reece146

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Not all ratios can be matched perfectly front and rear, depending on manufacturer.

I think the general rule of thumb is within 2-3% and you are good.

On an AWD vehicle this is why it is important to run the exact same diameter tires front and rear and at the same tire pressure.

Back in the ZJ days people used to complain about how short lived the viscous couple in the transfer case (NV249) was - I'm guessing a lot of premature failures were due to mismatched tires and pressures. If the tires are spinning at different speeds the difference has to be absorbed somewhere - in the transfer case in the form of heat and wear as well as tire wear.

In an XJ with AWD mode (NV242 transfer case) this was a non-issue since the centre diff is an open, mechanical diff. In the NV242 configuration, a mismatch between tires and pressures can make the transfer case difficult to get into or out of AWD mode which can lead to mechanical issues. I've experienced it in my XJ but haven't broken anything to date (~280,000kms).
 

1955moose

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Changing out both differentials sounds like it's going to get pricey. And theirs always a strong chance that something is going to go wrong, seen it too many times on this forum alone. How many miles on your 99 gunner? You might want to consider a second vehicle already set up the way you want it, and leave yours as is. I have a friend that has a 98 expedition 4x4 for $3400. Problem is we're just outside San Francisco.


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Snag

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Gunner,
As said by the other members yes you have to change them both out to numerically the same ratio and its going to be very expensive. The parts themselves are very expensive and they have to be set up properly so they wear and function correctly. Its just not a remove and bolt back operation.
Good luck
 

JoeD

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I think the general rule of thumb is within 2-3% and you are good.

On an AWD vehicle this is why it is important to run the exact same diameter tires front and rear and at the same tire pressure.

Back in the ZJ days people used to complain about how short lived the viscous couple in the transfer case (NV249) was - I'm guessing a lot of premature failures were due to mismatched tires and pressures. If the tires are spinning at different speeds the difference has to be absorbed somewhere - in the transfer case in the form of heat and wear as well as tire wear.

In an XJ with AWD mode (NV242 transfer case) this was a non-issue since the centre diff is an open, mechanical diff. In the NV242 configuration, a mismatch between tires and pressures can make the transfer case difficult to get into or out of AWD mode which can lead to mechanical issues. I've experienced it in my XJ but haven't broken anything to date (~280,000kms).
That 2-3% was the general rule of thumb for 4wd, and probably still holds true. It was common for a 4.11 rear and a 4.09 front to be matched from the factory.
But the point here is safety, and liability. Unless the vehicle had virtually been converted to 2wd exclusively. You just cant say I just wont use it.
 

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