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I am currently in the process of fitting a ford 8.8 with the necessary brackets to fit under the rear of my 01 Jeep TJ. In the interest of keeping cost down, I have decided to simply regear my front dana 30 to match. But before I run out and buy parts, I have a few questions. 1st, is that my rear 8.8 is a 4:10 ratio, and most sets I see for the 30 are 4:10 as well. I have been told that I have to offset my front to a 4:11 for some reason, but no one can explain why. all I've heard so far is that a 1% offset in ratio is "ok". is a matched ratio ok as well? if not, what do I do about this?

2nd, I believe that my current gearing in the D30 is 3:07. it is my understanding that I will have to swap to a different carrier to accommodate the 4:10 gearing. I have found carriers for roughly $100, but they are bare. What do I need in addition to the carrier and R&P in order to install into my housing? 

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Gary, one traditional view was that "gear bind" could occur in the transfer case if front and rear axle ratios matched exactly.  This is not much of a concern with a chain drive transfer case, nor was it ever a real issue, in my experience, during the heyday of gear drive transfer cases.

I run 4.56:1 at the front and rear of my Ram 3500.  I run 4.10:1 in the high pinion 30 front and OEM Chrysler 8.25" rear for the '99 XJ Cherokee, each axle in the XJ has an ARB Air Locker.  The earliest Willys/Jeep Spicer 4.88 and 5.38 ratios were fitted to models with gear drive transfer cases; worse yet, these were side drive designs even more prone to "gear bind".  Ratios matched exactly at the front and rear, and the outsourced axle supplier was Spicer.

The vehicle manufacturers have always matched front and rear gear sets when ratio matches were available.  However, when 4x4 vehicle manufacturers mixed outsourced front axles with an in-house rear axle application, the ratios often did not match.  As an example, Dana/Spicer might offer only a 4.10 front gear set, while the vehicle manufacturer had its own rear axle ratios, let's say the common 4.11 in the venerable Ford 9-inch axle.  A Ford light truck could have a Dana 4.10 front axle ratio and 9-inch Ford 4.11 rear axle ratio.

Rather than trying to offset gear bind, manufacturers were more likely living with the mismatch of axle ratios due to availability.  The fact that a Dana/Spicer 4.10 will work with 4.11 is to the manufacturer's advantage if they want to use an available and less costly in-house 4.11 rear axle.  3.92 with 3.90 was a fit in the day.  3.55 and 3.56.  Even 3.73 and 3.70.  4.86 with 4.88 could work, too.

Dana/Spicer and others use specific ratios due to axle casting design and fit issues.  They strive for uniform ratios in their products.  Typical Spicer/Dana front axle ratios are your 3.07, 3.55, 3.73, 4.10, 4.27 and 4.56.  These may be used by Jeep with a matching Dana/Spicer rear axle ratio or a very close match-up in the AMC axles.  In recent decades, manufacturers have introduced more uniform and industry standard axle ratios in products like your 8.8" Ford retrofit axle.

The better way to view this is just how much variance between front and rear axle ratios is tolerable.  I would believe that a ratio difference of 0.01 to 0.02, maybe 0.03, would be plenty.  In your case, be pleased that 4.10 is available in the 8.8" to match your Dana 30's anticipated and available 4.10 ratio.

It might be argued that aftermarket automatic lockers (Spicer/Dana Trac-Lok, Detroit, Tru-Trac, etc.) at the front and rear create bind.  They could if the traction surface is not loose and there is enough tire/wheel rotational slip to engage the locker(s)!  Front axles with OE limited slip were rare, we bought a new '85 Bronco II with an enticing Trac-Lok option at both the front and rear axles.  (I can go into how well that worked if anyone is interested.)  Ford used the same axle ratios at both ends of these 4x4 vehicles.  Obviously, front and rear full manual lockers, which function like a spool (think ARB Air Locker or Ox), should not be used on dry pavement or hard surfaces.

4x4 bind might be minutely offset by front and rear tires rolling at slightly different revs per mile.  However, ratio combinations like 3.07/3.08, 4.10/4.11, 4.56/4.55 or 4.88/4.86 might have less impact than tire diameter differences due to uneven tire wear or varied tire inflation pressures.  Wear per tire or under-/over-inflation can change tire diameters and revs per mile...Mismatched tire pressure could have just as much impact as these slight differences found in 4x4 axle ratio combinations.  

We could do the math to make a comparison...


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