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XJ 31's and 4.56 gears?

shimmy

shimmy

NAXJA Forum User
Location
seattle, WA
i have a 98 XJ, 4.0L inline 6. it is an automatic. does anyone run 4.56 gears with 31x10.5 tires? what RPM do you run at going on the freeway? any gas mileage differences from stock gears by switching? is this just an insane idea...? i have the gears already, but don't want to run too high of RPM's since i also use this as a daily driver. i plan on changing to 32" tires when the 31's wear out, but that won't be for a while...
ALSO, what is a safe RPM range for an XJ, i heard nothing higher than 3K...?

brandon
 
well, since 35's and 4:56 gets you back to stock gearing (I think?), you'll notice a HUGE difference in your driving. Offroad your truck will crawl along, on road, I bet you're limited to 65 mph.
 
I bet he isn't limited to 65. I went 80 mph for an hour and a half while going home from a trip one day. No problems.
 
offroadman83 said:
I bet he isn't limited to 65. I went 80 mph for an hour and a half while going home from a trip one day. No problems.
Click to expand...
wait, offroadman83, do you run 31's with 4.56 , or not...?
 
I use too. I shortly after 4 months upgraded to 35s and it sucked. 4.56s is nowhere near optimal for 35s with an auto. At least 4.88s or up to 5.13s would probably be much, much better. 4.56s was perfect for 31s IMO though!!
 
my daily driver is a 4.0L auto MJ w/ 4.10s and 235/75s

3300rpm @ 80mph, and it'll do it for 4-5hours straight (it'd go longer if I had a bigger tank)

that's still around 20mpg. I fawkin LOVE it.
 
shimmy said:
i have a 98 XJ, 4.0L inline 6. it is an automatic. does anyone run 4.56 gears with 31x10.5 tires? what RPM do you run at going on the freeway? any gas mileage differences from stock gears by switching? is this just an insane idea...? i have the gears already, but don't want to run too high of RPM's since i also use this as a daily driver. i plan on changing to 32" tires when the 31's wear out, but that won't be for a while...
ALSO, what is a safe RPM range for an XJ, i heard nothing higher than 3K...?

brandon
Click to expand...

I run 4:56's and 32's and will be jumping up to 285's (33x11.50) this weekend.
I know folks that run or have run with 4:56's/31's and they were pleasantly surprised with the outcome. I get better mileage with 4:56's than I did with 4:10's and I'm turning about 2800 rpms at 75mph.

Forget the lame gearing charts that scare you into undergearing your rig. They can't factor in rolling resistance of larger tires, increased wind resistance of a lifted rig, added weight of body armor, HD bumpers, full size spare, cargo, etc.

Bottom line......4:56's are great for 31-33's tires, but 4:88's are better for 33-35's. Do it right, buy gears once........not 3 times like some of us...........
 
4.56 with auto and BFG MT 32s

I am running 4.56s with auto and BFG MT 32s. The reason I mention the brand of tire is the BFGs tend to run small for a 32. I am running about 2850 at 70mph. with a 31 you should run 150 to 200 rpm higher than that.

I drove from NC to Moab for NAXJA event last fall and ran all day at 2800-2900 with not adverse effects. Got about 16-17 mpg fully loaded.

I am of the opinion from reading many posts that 2400-2500 is the sweet spot for the 4.0l, so I am a little higher than that but not much. I am going to 33s soon so that will drop my rpm about 200 for the same speed. Should put 70MPH at just about the right place.

Around town and wheelin the 4.56s are a lot nicer. Had 3.55s and truck did not do well "lugging" around.
 
So, I take it that If I am to run 31's then it would be a good idea to change my stock gears to 4.56? I am a noob at do-it-yourself modifications, so I value everyones input because I fugure its better to learn from experience.

I was going to just buy the lift and tires and put them on with no other changes. What are some suggestions on what I should buy (besides the suspension and tires) to get my 92XJ Sport to be well equipped for the lift and tire size increase? I am sure there are a million suggestions throughout the NAXJA forums but that would take hours to read them all! I appreciate anyones time and suggestions!
 
Tow points front and rear, High lift, Recovery straps, Armor, Fire Ext., First aid kid, CB.

This is a basic list of the stuff needed to have by yourself or with a group.
 
in my haste to get a hitch, I bought a reese class 3. The problem is that the design I bought, the reciever doesn't stick straight out of the mount, rather there is a drop and then the reciever sits below the mount, so I had to get a ball mount rise. I am not about to take the hitch off after the trouble I had putting it on. I figure a 3"lift might help as well!! LOL
 
OK, so BTT, i also have a BPI bumper with winch, rocksliders, and rear four real steel bumper. from what everyone is saying, i should be fine running 4.56 gears with 31's then? They are MTR's.

brandon
 
motorman said:
I am running 4.56s with auto and BFG MT 32s. The reason I mention the brand of tire is the BFGs tend to run small for a 32. I am running about 2850 at 70mph. with a 31 you should run 150 to 200 rpm higher than that.

I drove from NC to Moab for NAXJA event last fall and ran all day at 2800-2900 with not adverse effects. Got about 16-17 mpg fully loaded.

I am of the opinion from reading many posts that 2400-2500 is the sweet spot for the 4.0l, so I am a little higher than that but not much. I am going to 33s soon so that will drop my rpm about 200 for the same speed. Should put 70MPH at just about the right place.

Around town and wheelin the 4.56s are a lot nicer. Had 3.55s and truck did not do well "lugging" around.
Click to expand...

Sweet spot varies with the motor, the pre-HO 4.0's peak at a lower RPM than the HO's..........which is why I couldn't run you down when we were coming off of Spike..........didn't do too bad for an ol' 183K mile mill. ;)
 
how would running 4.56 and 31 work on a 89 4.0 engine with 168k and some blow by? And I get more blowby with highway driving then anything. I'd be worried about running that high of revs on a older engine. Opinions?
 
Getting ready to go 4.56's w/ 31's myself - keep in mind if you're running an AW4, most gear calculators aren't factoring the generous 0.75 drive ratio the AW4 has in 4th/OD - so their RPM numbers are gonna be skewed upwards by a couple hundred RPM.
 
Yeah, 4.56s is mostly back to stock with 33s. And 4.10s for 31s.

Thats not stopping me from going with 4.10s with 33s. :( lol.
 
I agree. I run 32's and 4.56's on my DD/huntin '96 XJ. I think it'd run great with 31's too. I regeared twice before I got to the 4.56's.

XJEEPER said:
I run 4:56's and 32's and will be jumping up to 285's (33x11.50) this weekend.
I know folks that run or have run with 4:56's/31's and they were pleasantly surprised with the outcome. I get better mileage with 4:56's than I did with 4:10's and I'm turning about 2800 rpms at 75mph.

Forget the lame gearing charts that scare you into undergearing your rig. They can't factor in rolling resistance of larger tires, increased wind resistance of a lifted rig, added weight of body armor, HD bumpers, full size spare, cargo, etc.

Bottom line......4:56's are great for 31-33's tires, but 4:88's are better for 33-35's. Do it right, buy gears once........not 3 times like some of us...........
Click to expand...
 
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