another 8.25 swap conundrum

[luckycamper]

Hey guys, after reading post after post related to the 8.25 axle swap, I, a novice, still have some questions. 

 

I am planning on doing a 4.0/ax15 swap in about a year and ditching this 2.5 and with that, planned on swapping out the rear axle too. As fate would have it, I’ve come upon an 8.25 (3.55gears) out of a 98 cherokee for $125! The only thing is, I believe I have 4.10 gears(?)—bc I’m rocking the 5speed, have to make sure about this first. btw If anyone has a hack to identify gearing ratio sans cracking open the rear diff lmk.

 

So my main question is, with future engine swap in mind, would I be loosing more than gained, if I swapped in axle with higher gears. Btw i’m running 265/75r16 tires. 

 

 

 

[ghetdjc320]

If your planning on keeping that tire size 3.55’s are ok. Basically you have a metric 31-32”. It won’t be the best gear ratio if you intend to off-road or you have a lot of hills to deal with but it will work. I’d suggest 4.10’s for that tire size.

[luckycamper]

yea I live in Montana so there’s a fair share of hill climbing once you get out of the city.

 

thinking I will pass on the deal for now. Not trying to rebuild a rear diff especially before I open the pumpkin and verify gears…

[gogmorgo]

You can get a pretty good idea of gear ratio by lifting one rear wheel off the ground, and counting the number of driveshaft rotations required to turn it two full rotations. Just over four driveshaft rotations will be a 4.10 ratio, 3-3/4 will be 3.74, etc.

 

So long as you're reasonably gentle with it, a d35 will hold up to 31's for... an amount of time. They definitely aren't the toughest axle out there, but they aren't exactly made out of glass, either. As best as I can tell the one under my longbed is still original at over 300,000 miles, so as long as it's not worn out, it should hold up long enough to either find or build the axle you want to run, so long as you're not abusive to it, although I'm setting a pretty low bar myself on gentle treatment. 3.55's are also more than tolerable, but they're definitely on the taller side for 31's. They're OK for a daily driver, especially one that sees lots of highway miles, but if it's a dedicated offroad toy you'll want something deeper for sure.

[Eagle]

21 hours ago, luckycamper said:

Hey guys, after reading post after post related to the 8.25 axle swap, I, a novice, still have some questions. 

 

I am planning on doing a 4.0/ax15 swap in about a year and ditching this 2.5 and with that, planned on swapping out the rear axle too. As fate would have it, I’ve come upon an 8.25 (3.55gears) out of a 98 cherokee for $125! The only thing is, I believe I have 4.10 gears(?)—bc I’m rocking the 5speed, have to make sure about this first. btw If anyone has a hack to identify gearing ratio sans cracking open the rear diff lmk.

 

So my main question is, with future engine swap in mind, would I be loosing more than gained, if I swapped in axle with higher gears. Btw i’m running 265/75r16 tires. 

 

 

There's often a tag on the front axle with the gear ratio. Even without the tag, a 2.5L with a 5-speed is almost certainly going to have 4.10 gears.

 

Your tires calculate out to a diameter of 31.65", which is (as ghet has noted) the metric equivalent of a nominal 32-inch tire. Even with a 4.0L engine, most folks want at least 4.10 gears with that size tire.

[luckycamper]

Didn’t find a tag on the front axle so I’ll be lifting up a rear wheel and spinning the drive shaft tomorrow. After some consideration and hearing from you guys I’m hoping it is indeed 4.10 gearing but if it turns out to be 3.55, operation 8.25 shall commence. Unfortunately I have to replace u-joints this week and it seems like I have a pinion seal leak on my hands so either way October has become driveshaft/axle month.  

[Pete M]

just remember that if you rotate a single tire, it must go around twice.  if the axle has a posi, then you'd need to jack up both rear tires and spin them together around just once.   

[luckycamper]

Jacked her up, spun a wheel, and to my dismay it was just shy of 4/2 so it seems like perhaps a 3.73. darn

[ghetdjc320]

3.73 is a doable ratio for your tire size. 

[75sv1]

Since your in Montana, I think the 3.73 might work out better for you. My experiences are with auto trans and WV. On The same tire I have 3.73's with an auto. A bit weak on some mountain inclines. Intake and exhaust improvements help. This is on a 2000 XJ. I still have a few inclines that I need to drop down from D to 3rd. On a '98, I had a similar problem, with 4.10's and 31's. I did basically, the same exhaust intake improvements. It ran good. 

        So, since I think you will have some mountain passes to get over, I think the 3.73's with your manual would work out better. You might loose a bit of MPG. my though is 1/2 MPG.  My intake and exhaust improvements are, bore to 60mm TB, 2.5 cat back, 99+ intake and a header.

[75sv1]

Also, I'd expect you to be at 2300 rpm at 70 mph with 3.73's and the manual. I'm at 2200, with an auto.