[QUOTE=96_xj;649525]bte sorry she got in the picture but.... here is her's

Does you Ex know your exploiting her . . . lol, just joking 96

[QUOTE=dans01classic;649535]yes, yes she knows she is on this forum involuntarily lol. she is cool with it

The way I figure out how far off my gearing is from idea is to check ratio between the stock configuration and where you want to go with it.

For example, my thought was to start out with 33s, and then go to 35s, but I was worried about having to change gears for each level. I figured I'd do the math and find a happy medium.

Stock gearing (for my jeep) is 3.55 with 29.5" tires.
So, if I'm going to 33s, I'm increasing by 111%. This would mean my gears should be 3.97:1 if I want the jeep to perform exactly the same as before, if they made gears in that ratio (nearest is 3.73, which would be anemic, and 4.10/4.11, which would give a little more pep on accel/hill climbes, but higher engine RPM at highway speeds)

If I were to do the same for 35s, it's a 118.6% increase, resulting in 4.21:1 ideal gearing (if stock is considered ideal for 29.5" tires).

As a result of these figures, and searching for what gears are availible for my axles, I had figured on getting 4.11 gears (aka 4.10).

BUT...

The last trip out tought me something. Before, I'd been hot-dogging my stock rig in mud or mostly flat trails, not much of actual crawling. My style of driving was pretty much in the reasoning that I'm slipping at all times, keep the RPMs up in the powerband, and slog on through. Last time out, though, it was mostly dry hill climbs, which I found my jeep's overall performance to be a little too low on the gears. Aside for more torque with higher gears, you also gain control, and if you can match what kind of driving you're doing so what you do is in the powerband, you have better performance overall, at the cost of speed.

As such, I'm looking at 4.56 or 4.88 gears instead, even if I only go up to 33s for now, as I'm not going to be doing any highway stuff after that. This, I feel, will give me way better control on the more difficult climbs, especially on things like sandstone climbs, where its firm but wants to break loose. I'm not wanting it necessarrilr for torque, but for overall performance.

If you're mostly just street driving and wanting to figure the best tires for gearing, figure out the neighborhood you want to go with (with the above, it would be 4.11:1) and do the math:
4.11/3.55=1.1577464 (115%)
1.1577464*29.5=34.15"

So, the ideal gear/tire ratio in the 33-35" range is 34" with 4.11 gears.

For yours..., well, you'd have to figure out your stock gearing (I verified mine through a bunch of different sources, but today I looked it back up with searching for "xj" on wikipedia. It came out the same) and stock tires (235/75R15=29.5" tires, approx.). That ratio was what your jeep was engineered for. If you're happy with the 31s you're driving now and aren't missing out on any power, figure out the increase and try to think if it sounds like you'll miss that much power. This is done by inverting the numbers:

(if you ran 34" tires with stock gearing) 29.5/34=.867647 (87% of origional torque)

I guess looking at it this way, right now you're at 29.5/31=.9516, or 95% of origional "get up an go" torque, which the I6 has plenty of. Bumping it up to:
32"=92%
33"=89%
34"=87%
35"=84% origional torque at the wheel at a given engine output (RPM & horsepower).

Anyway, the big thing to keep in mind is that as the tire size goes up, the engine, transmission, transfer case and whatnot all have to work that much harder to maintain the same speed without downshifting. More work= more heat, and more heat= more trouble overall. Mileage goes down, overheating issues can increase, and so on. (also known as lugging)

Of course, if you go the other way, and overgear it, like I'm looking at, you also lose milage on the flat as you have to keep your RPM up. Workload goes down overall when you're under load, but driving on the flat you are trying to keep RPM and torque BOTH down (roughly speaking, RPM*torque=horsepower, the more horsepower used, the more gas is used), so while you're better on the trail, that's about all you're doing better on. Also, if you're gears are too high, you have just multiplied the torque at the wheels, as well as the axles (all gear ratios in the system multiply together for final torque), and therefore run the risk of more breakage if you use that power.

Hope something in that helps...

^^^
This sounds like gearing ratios for a Wrangler...
Check out this chart: http://www.4lo.com/calc/geartable.htm

I dont know about him, but that helped my a ton! Thanks TrollHammer, i guess im going with 3.5 inch with 32's on stock gears. Jeep only has 82k on it so the trans is healthy enough to take it.

thanks a lot guys i have an automatic 96 with the stock axles so it looks like il get the 32's and when ever i have the time start working on an axle with 4:10s. i dont plan on going any higher then 33's on my xj and it will see mainly mud.

I just put 33's on mine from 31"s and loss of power is very noticeable for each size with stock gearing. It is probably just from being up in the rockies with very steep climbs on the trails and road. On the interstate with 31's even I had to downshift my automatic to the 1-2 range just to get over Vail pass. If I didn't I would slow down to around 40 MPH and dog it up the mountain. And on the trails, sometimes even in 4low I get to the point that my XJ is going so slow it just stops with the pedal pinned to the floor. I half to put it in neutral to get some RPMs up and throw it in drive to get over steep sections. I have been putting off getting new gears for 2 years now but since I got my 33's I didn't have an option and now swapped my gears to 4.56s and it is a night and day difference. I can stay in drive continuously on road and trail and it climbs like a champ every time. Everyone has there own opinion as you can tell but I say buy the tires and upgrade your gears as soon as possible when the funds are there.

That illustrates it a lot better, and as far as the gear ratios being like a Wrangler, it would be for two reason IMO: much of the 4.0 wrangler is built like a cherokee (drivetrain wise), and because the same principles generally apply to most vehicles in the same class (weight range, RPM power range, engine output). Thanks for the link!

I look at a lot of Wrangler power train stuff. Currently waiting for some down time to put in the '99 intake and injectors from a wrangler in my '91 cherokee. There's some wiring differences between the two, but mechanically they're close to the same (and identical as far as the intake thing goes). I believe the wrangler is lighter, but that doesn't change the engine performance (powerband), only what gear ratios they might throw in a trans.