4.88 on 31s too low?

[4x4Phil]

Hi guys,

I'm doing a conversion for a friend from 2wd to 4wd for a 99 4.0 aw4 auto and just sourced the hp d30 front axle. If I'm right the d30 has 3.73 gears and the xj currently has 3.55 in the c8.25 rear.

I was thinking of swapping a new ring and pinion 3.73 in the rear to match the new donor axle or I can buy a kit and go higher.

The final tire size will be 31" with a 2.5" lift. I have a set of 4.88 gears for both axles but am worried this is going to be too low as this will only see off-road about 10% of the time and be a commuter the rest of the time.

4.88 crazy? Better to swap rear to 3.73? Or buy a set of 4.10s?

Thanks

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[RCP Phx]

Way to low for 31's!

 

[SlowXj]

4.88 would be quite low... I have a very tired (weak) xj and I loved 4.10 and 31s.... drove great, handled 80 with ease... the power was better than stock imo....

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[Runnin'OnEmpty]

Here's another vote for 4.10s.
My Ranger has 4.10s with 30.5" tires, and that
combination is about perfect.

 

[Evan03]

3.73 will be pretty much the same as 4.10. Do whichever is easiest to get axles matched

4.88 will have her screaming though I'm not sure she'll be screaming for more

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[75SV1]

My present XJ is on 31's with 3.73s. I planned it for MPG and mainly highway driving and some DD. Maybe some trails or dirt roads. It does have some issues with the mountains in WV. Not so much on the PA Turnpike though. I do have a 2.5 exhaust and bored TB. They help.
One of my other XJs, had 31's and 4.10's. I also has 2.5 exhaust and bored TB. The bored TB really made a difference in the WV mountains. No down shifting. Its on 32's now. A bit sluggish at times. I did wheel it with the 32's at Winterfest. I did ok. More had problems with the mud.
I'd go with 4.10's, if there is some planned wheeling or off road. You might lose a .5-1 mpg. Still, other ways to get that back.

 

[IslanderOffRoad]

4.10's would be ideal, 4.56 would be about as high as I'd go- and I'd only do that if you were planning on bigger tires down the road.

 

[maxbraketorque]

After having primarily street driven 31 inch tires with the stock 3.55 gears for a bit over a year, I feel that 3.73 gears would work fine for an XJ that is not seeing hardcore offroad. In fact, 3.55 gears get the job done. 4th is for flat or downhill highway above 55-60 mph. Everything else gets 3rd gear. This works quite well because the TC will lock 3rd gear when the shifter is in third, and the 3.55 ratio produces about 2500 rpm at 60 mph on an uncorrected speedo. Its even possible to use drive for all but highway hills, but for sure highway hills need to be driven in 3rd gear. 3.73 would make 4th a little better on flat highway and raise rpms very slightly in 3rd gear which wouldn't be a bad thing either. If you're trying to save time and money, I say toss 3.73s in the rear and be done.

 

[denverd1]

unless you're towing several thousand pounds on a trailer through colorado, way to deep

 

[SlowXj]

As another option... I the front is 373 a rear 8.8 373 is pretty easy to find... I think my 8.8 swap was about 300 bucks when all said and done.... strength improvement, lsd, disk brakes...

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[75SV1]

As another option... I the front is 373 a rear 8.8 373 is pretty easy to find... I think my 8.8 swap was about 300 bucks when all said and done.... strength improvement, lsd, disk brakes...

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I went this route on my 2000. Also, did this with 4.10's on my '98. On the 2000, I used a ZJ front with 3.73's. They are low pinion though. I kept the ZJ's front axle adjusters, but upgraded to RE lower control arm brackets. I had a set of Artecs for it, but will go on another build. For 4.10's, some XJs 4cyl manuals have 4.10's. The front axles can go for $600 though. I think the rears about $150. Maybe a lunch box locker and KKJ disc?
Also, I put on K-Suspensions mini trust and upper control arm bracket stiffener and C-frame stiffeners (Dirty 30), plus Stinky Fabs spring bracket stiffeners.