Hey guys, first off let me say this seems like a pretty cool bunch of peeps!! I just bought my 90XJ 2dr, 2WD and was wondering what rearend i have and what gears are in there, I bought the XJ from some lil old lady that didnt know anything about it. I already bought some 31 x 10.50 x 15's for it that will be going on this week. I also did a complete tune up to it, she's running great, It runs great consideing how old it is. Thanks Any help is greatly appreciated!!!

 

post a pic of the diff so we can tell what axle you have. on the gear ratio it just depends.

 

Thanks man, I just uploaded one onto my albums, thanks for the help!!

 

That's a Dana 35.

 

Thanks!! So now my question is what gear is stock and what is recommended with a 31 inch tire?

 

yup, thats a dana 35 that tag that is on there should have the gear ratio on it.

 

Ok i'll crawl under there and take a look. Whats recommended with the 31's or shoud i just ask the guys at Pro-Gear where im taking the Jeep to have the diff gone thru?

 

It should have 3.07, 3.55, 3.73, or 4.10.

 

So, if it's running great, what's ProGear going to do? Looks like a new diff cover gasket and fill plug are in order. U can do that cheap and easy. U can almost bet a shop's going to find something wrong......chaching, chaching.

 

Im going up in tire size so i am thinking of doing gears next to compensate for tire size, your right gasket and fill plug are an easy fix, I trust the guys at Pro-Gear, they race with us and we've been dealing with them for 15+ years.

 

I'm running 33's on stock gears.. My rpms are a little high at highway speeds, but certainly nothing I can't live with. If your not going any bigger than 33's I would honestly just use the money you would be spending on gears for something more useful. I know a lot of guys that run on 35's with stock gears and don't have any problems. I would certainly not put gears on the top of my list. Check the tag on the diff cover bolts for the gear ratio; it's most likely 3.55.

 

Wouldn't rpm be lower with taller (than stock) tires and stock gear ratio?????????

 

Yes because its harder to push bigger tires so gears are to compinsate for the extra tire

 

Yes the rpms would be lower, a BFG KM2 size 235x75r15 revs per mile is 715, a BFG KM2 size 33x12.50r15s revs per mile is 634 so the engine would be turning much lower rpms with the taller tires

 

I'm not an expert on the topic, but I do believe your understanding of engine rpms versus tire rpms is incorrect.

It takes more revolutions of a small gear to turn a large gear. When the size of the larger gear is increased but the small gear remains the same, it takes more rotations of the small gear to fully rotate the large gear. When you change to a lower (Numerically higher) gear ratio it increases the size of the small gear, which will then make it require less turns of the small gear to fully rotate the large gear.

Basically what I'm saying is, with larger tires on stock gears, your engine rpms will be increased while you are driving compared to stock size tires.