2000 xj aw4 on 34s 4.88 or 5.13

[Andre2000]

hi all looking to throw some diff gears at my xj just wondering were my rpms are going to end up , I've got the aw4 trany running on 34inch bsl super swampers
current gears in diffs 3.55

the jeep is not a daily driver but i still wanna hit 65mph on hihway and be around 2500rpm max

was thinking 4.88 or 5.13 looking to get as much power as possible and still hit 65 comfortably

also not my front dana 30 is a low pinion and they do sell the 5.13 gears for it

 

[RCP Phx]

I wouldn't do 5.13's! 4.88's are going to limit your top speed, I would even consider 4.56's.

 

[Andre2000]

what do you think top speed would look like on 488 ? and were do you think the rpms would sit ?

 

[RCP Phx]

4.88's and true 34"s should figure out about 2900-3000 @ 65mph, there are plant of calculator out there that can exactly.

 

[RCP Phx]

"Plenty of calculators"

 

[RCP Phx]

My rough calc came out to 2500@65

 

[Andre2000]

So I measured my tires and they actually only measure 32 1/2 on the tape while on the Jeep so I guess bsl 34s aren’t acually 34s ?
Would 456 give me a good balance ?

 

[RCP Phx]

For a accurate number remember to use your loaded circumference (not free-air) and your od is .753, at whatever your normal tire pressure is.

 

[Andre2000]

That measurement was under load of the Jeep
What is od.753?

 

[RCP Phx]

You need the loaded circumference to be accurate. OD is overdrive ratio.

 

[RCP Phx]

Also you want to take that measurement with the tires at full operating temperature!

 

[Andre2000]

Darn won’t be able to take that measurement lol got the rear diff all ripped appart
Ring gear bolts out of the 355 all stripped when I tried removing them to install my Detroit locker wich is how I ended up looking for gears

 

[RCP Phx]

Then use your measurement and keep in mind any tire changes in your future.

 

[Andre2000]

456 is starting to sound like the better gear ....

 

[RCP Phx]

For 4.88's and 32.5 I came up with 2470rpm!

 

[RCP Phx]

For 4.56's I get 2307rpm.

 

[Andre2000]

Man that’s bang on 488 is right we’re I wanted to be then

Thanks for the info much appreciated!

Think I’ll order up 488

 

[RCP Phx]

I don't think you can go wrong with that choice. I've got the best of both worlds (street/off-road) with a 4:1 Tcase and the plus of 2low.

 

[Andre2000]

What tcase did you drop in ?

 

[maxbraketorque]

Unless your XJ has significant power mods, its going to be somewhat doggy a 2500 rpm at 65 mph with those tires and necessary lift. TC will be unlocking or trans will be downshifting on a mild incline. 5.13 gears will get the motor up to 2600 rpm at 65 mph which will be better. That's not much higher rpm and will be less likely to unlock or downshift.