tkdtim

So whats the recommended gearing for different tire sizes to bring the power band back to stock or better. I have 31's and 5th gear is a little sluggish and holding 70 on the interstate against the wind can be a chore.

stock tires = 3.07?
31's = 4.10?
33's = 4.56?
35's = 4.88?

4.3L XJ

4.56 will work with 35s too

BuckB91XJ

TKD, you have the perfect tire to gear chart. Go with what you have. 4.56s will work for 35s but if you're gonna do more driving around town and 4wheeling than driving at highway speeds. The 4.88s will be better for 35s.

EndlessMtnFab

Perfect ratio depends how you want to use the Jeep.


The generally accepted formula is:


New Tire size
___________ X Current Gear ratio

Old Tire size



So 31 / 28.5 x 3.07 = 3.33



That said ... 3.55 is really the best ratio to get you back to "where you were". Auto guys ... don't forget his five speed has a higher overdrive than you (meaning it doesn't drop RPMs as much). I ran 3.55 for a few years with 30s and 5 speed .... at the time, it was my wife's DD. And at 75+ mph the motor starting climbing in RPMs.


If this is only an occasional driver and mainly trail rig --- then I could suggest 3.73s or 4.11s. If you're still trying to keep it as a DD ... I would probably stick to 3.55 and keep 3.73 as the "extreme" end of the deal.

Yes ... regearing for not a huge change can be seen as a bit wasteful ... but sometimes the obvious isn't



Just my opinion



Joe
EndlessMtnFab

mike37

you have 307 gears in a cherokee stock ??

EndlessMtnFab

4.0 & 5 speed in an XJ was *usually* 3.07s


I *assume* that is what the OP has.


High Altitude Packages (like those sold in CO) got 3.55s with their 5 speeds. Also received the offset CPS for greater timing.



Joe
EndlessMtnFab

tkdtim

Ya, I currently have 3.07 running 31's.
I think it runs about 1600 RPM at 55mph in 5th. I could basically drive everywhere in 4th.

The jeep is mostly used as DD. Would you guys look for a better ratio like 3.54, 3.55, 3.73, 4.10 range?

The local pick n pull has about half a dozen jeeps so I might be able to find a set of matching axles.

EndlessMtnFab

My vote is either 3.55 or 3.73 ...


Depends what you find and how much hiway fuel mileage you are willing to sacrifice.



Joe
EndlessMtnFab

ol"blue

I would've said for a 31" tire go with 4.11s but I guess it's not right for you. In your case, if you can get both front and rears I'd go 3.73s, or 3.55s. JMO

mike37

don't see a lot of 307 gears in a Cherokee mostly 355 and 372
the ax15 has the same over drive ratio as the aw4
the aw4 has a lock up converter so 307 would be really boggy gears in any jeep

thecj7 with the 304 v8 had 307 gears if I'm remembering right

EndlessMtnFab

Mike ... this is a 5 speed, You don't see many Cherokees with 3.55 or 3.73 with that ratio you have a standard trans. If you have an auto ... those ratios are quite common.


And the AX15 overdrive ratio is .79 (21% drop) The AW4 overdrive ratio is .75 (25% drop).



Joe
EndlessMtnFab

mike37

1600 rpms is way to low for the 4.0 to have any power
3.73 would get you back in the power band
4.10 would give you a great low gear for of road

mike37

drop or bump up ??

EndlessMtnFab

3.73 would put you at 2300 RPM at 70 mph (assuming speedometer is correct in the first place).


4.10 would place you at 2500 RPM at 70 mph (see above disclaimer).



Joe
EndlessMtnFab

EndlessMtnFab

Drops your RPM by 21 or 25 % respectively.


If your RPM's went up .. that would be an Underdrive unit (commonly used for HD towing in the mtns -- think campers or trying to drive with mother-in-law in the bed of the truck)



Joe
EndlessMtnFab