Gear ratios and 4WD

I just picked up an XJ D44 with 3.54 gears. Up front I have 3.55's. That difference won't be a factor off road but I suppose this means I can't use full time 4WD (NV242) on road anymore, correct? Or at least until I regear.

--Matt

Yes, you can. It's the same ratio.

Different axles have different size ring and pinion gears, so the tooth count is different. The ratio is the number of teeth on the ring divided by the number of teeth on the pinion. Then they round it to two decimal places. It's almost impossible for two different axles to work out to exactly the same ratio.

Between 3.54 and 3.55 -- you'd see a bigger difference if you ran the tires on one end a few pounds softer or harder, and you certainly create at least that much difference by going around a corner. Put that sucker in 4-full-time and have fun.

egon, I have a 1987 XJ, and I am still running the factory axles and diffs. The tags on the diffs read, 3.54 in the front D30, and 3.55 in the rear D35. That is what the factory starts out with, so I doubt you have anything to worry about. Run it, it is correct.....

Casper

As Casper stated, he has 3.54 gears in his D30 whereas I have 3.55s and the vehicle that donated the D44 has 3.54s up front. What is the difference between the two gear sets?

Casper, which t-case do you have?

--Matt

One of you doesn't have the ratio you think you have in the front. The Dana 30 only comes with one gear set in that ratio. I don't remember if it divides out to 3.54 or 3.55, but it doesn't matter. As I explained, they are different only because a Dana 44 has a much bigger ring gear than a Dana 30, so the ring gear has more teeth. The pinion gear also has more teeth. Count the teeth and do the math and that's why the ratios are "different."

In practice, they are the same. That's the way they came from the factory. Whatever you have in the rear now -- Dana 35 or Mopar 8.25 -- I'll bet if you count teeth and do the math it won't be the same as the front. This is why with a part-time transfer case you're not supposed to dive on pavement in 4WD. With the full-time transfer case, the internal differential allows for that.

As I posted before, when you go around a corner the front wheels swing a wider radius than the rear wheels. That alone accounts for more of difference between the front and the rear than 1/100 of a point in actual gear ratio.

Don't make it more difficult than it is.

Eagle said:

Don't make it more difficult than it is.

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Agreed, but for some reason I have a propensity to contiuously be a pain :fuse:

One of you doesn't have the ratio you think you have in the front.

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I can't speak for Casper, although. The 30 on my '96 have 3.55 gears as per the tag. The 30 on the '90 I got the 44 out of had 3.54 gears according to to the tag. I specifically looked at the front tag because I couldn't see the tag on the rear until after the axle was pulled. My best guess is that 30s paired with 44s get different ratio tags than those paired with 35s and 8.25s or someone desided to use "The New Math" one day over at Dana.

Yes, I'll shut up now.

--Matt