axle ratio formula

[creekscout]

I would like to know what the formula is for determining rear axle ratio. Also is there a way to figure out what it will change to with different size tires?

[87manche]

gojeep.willyshotrod.com

 

look under gear chart.

[feerocknok]

I always send people to http://www.rocky-road.com/calculator.html.

 

Put in your stock gearing, stock tire size, and new tire size, and it calculates gears to keep your vehicle performing like it did stock.

[creekscout]

Thanks for the links on how to recalculate the ratio, but the first part of my question is how to find my trucks actual factory ratio. The reason for my question is that I am pretty sure it must be either a 4.10 or 4.56 (D44) because of the high rpm's at cruising speed. If that is the case I would like to increase my tire size to get to around 3.73.

[aemsee]

Jack up the rear axle, using two jack stands, making sure both wheels are off the ground. Check to make sure they both turn freely. Make a easy to see mark on the drive shaft and one on the inside of a tire where you can see it from under the truck. Now turn the shaft while counting how many times you need to rotate the DS to have the tire make one revolution. 4 DS turns will be about 4.10 and 4 1/2 turns will be the 4.56. Make sure both rear wheels turn so you are not factoring in the spider gear ratio. OR, just pull the cover divide the number of pinion teeth in to the number of ring gear teeth. You probably need to change the fluid anyhow, right? :D

[87manche]

as far as I know 4.56 was only in 2.5L trucks equiped with the aw4, a rare find indeed.

If you've got a 2.5L with a 5 speed it will likely be 4.10

2.5L with a 4 speed=3.55

4.0-auto=3.55

4.0 5spd=3.07.

General giudlines, there were some factory 3.73 axles, but it depends on how your particular truck was optioned.

[Pete M]

There are no factory 3.73 MJs until someone can prove to me otherwise. 3.73s were found in ZJs and a few XJs with the tow package.

[BrettM]

There are no factory 3.73 MJs until someone can prove to me otherwise. 3.73s were found in ZJs and a few XJs with the tow package.

 

i've seen it in a LOT of XJs at junkyards, and I think some MJs, but I can't say for sure.

[Eagle]

I have an '86 XJ 2.8L automatic (or what's left of one) that has 3.73 gears, but I don't know if they were ever offered in the MJ. The hulk I have in the yard is the only XJ or MJ I've ever seen that actually has 3.73s in it.

 

And I don't know if the front axle is even original or matches the rear. It was a hulk when it followed me home, and once I started poking around I began to notice a few -- anomolies.