I just bought my XJ which came with 31" tires. I recently found out that even though the previous owner added larger tires, he did not regear his differentials. I've been noticing a few issues with the driving and was wondering if this could be the culprit or is the overall performance not affected that much by the gear ratios? Also, if this is an issue, would it be easier to buy smaller tires or regear the differentials?

 

Absolutely.. gears matter a lot and are not easy to do w/o propert tools and training. BUT, is definitely worth the investment of time and money if you have bigger tires and want to leave the pavement.. ooooya
I have been told.. that the easiest thing to do is (if you have a 6cyl.), get the stock axles out of a 4cyl. which will have a steeper gear. Less HP requires steeper (added torque) gears. So, put them in a 6cyl. and bigger tires (like 33s) are a pretty good fit. (Note: your speedometer will never be right again) If you stay with 31s, the 4cyl. axles will give you more torque, poorer fuel econ. but get you crawling slower wich is better for off road. So, if you have a 4cyl., you need to re-gear or stock tires...

best of luck

 

With 31's its hardly worth the expense to regear. I just upgraded to 31's, before I did I searched on here and most advised that it was'nt necesary. I noticed no loss performance wise since lifting it and adding the bigger tires. Just a FYI, your speedo is going to read slower than you are actually travelling due to the increase in tire diameter.

 

I agree the 4.0 does just fine with stock gears and 31" tires, alot of $$ to re gear and a loss of fuel economy.

 

if you run a 5spd you will want to regear... auto you may not be too bad.
Deeper gears do not always mean worse fuel ecconomy.
I run 4.56's and knock down high 20mpg-low 21mpg on the interstate (65-70mph) in mountain terrain.

You can easily correct you speedo by purchasing the correct speedo drive gear. You'll pay less than $15 for one, and the install involves 1 wrench, one pair of needle nose and about 15 minutes.

Gearing the diff's is cheaper than buying quality (BFG/Michelin/Goodyear/Toyo) tires... especially if buying 5 tires.

 

Can I ask where you found a speedo gear for $15. I've been looking and all the ones I find are considerably pricier, thanks.

 

dealership probably

 

Alright, thanks. I'll start researching different gearing options and prices. Any recommendations? I was thinking on going up to a 4.10 with my previous Wrangler but would a 4.56 work better for the XJ? I primarily use my jeep to get around Atlanta but I do get on trails pretty often.

 

X2, $14.00 at my local dealership...

 

Hey just a question i was planning on running 31's and was wondering should i get 4.10's ?Also its my DD and i do mostly highway driving and it would be nice if i got some better mpg.And would it be cheaper to regear them or swap them out for 4 cylinder axles?

 

I say find a buddy that has interchangeable parts with your rig but has a 4cyl. and who who has/wants bigger tires. You get his axles, he gets your axles and you split the cost of re-gearing once rather that twice. (If im not mistaken 6cyl. have 3.73 & 4cyl. have 4.11) PLEASE confirm before doing anything...
http://www.izook.com/gearcalc.htm This is the gear calc. that I used in the past.

This should be very easy.. And approaching someone you do not know would be okay too, they will probably thank you.

Good luck and dont forget to start a thread and update it with each step.. it could even be an advertisement
examp.(1991 4.0XJ wants 91-99 4CYL. axles for exchange = split cost of re-gear 4you too = Columbus, Ohio)

I now run 91XJ w/4.56 and 35s (should have taken my own advise.. Hindsight is 20/20, right?)

Best of luck.......

 

OK thanks will do that but 4.10's will give me better gas mpg right?

 

I forgot the application.. but with 6cyl/inline6, auto trans, 31s.. (I checked your post)
4.10 & 4.11 are so close its negligible.. fuel economy might equal.. they are better to get the tires turning (torque) but less high end speed, thereby making your hwy cruzing RPM higher on average.. consuming more fuel.. but made up with the larger tire size and the longer travel before a full rotation.. think of a record player.. the outside edge travels faster than the inside.

Mike H. said that he runs 4.56s and does okay but he has a 5speed... if he ran stock 3.73s.. he would burn a clutch getting those bigger tires moving

I run 35 inch tires and have a 5 speed transmission with an inline-6 & 4.56 gears.. I want to have max torque while keeping it somewhat street-able... on a down hill in the woods, I want the gears in 4lo, in 1st gear to keep me at an absolute crawl.. the slower the better... I hope that is the case, the build is not done yet.

You with 4.56 gears, 31s and an auto trans would chirp the tires and need a kidney belt with the off the line torque.. or at least I hope you would.. if not.. my application will require a regear which I would do an Atlas II transfer case upgrade... but who has THAT kind of money?! I sure dont... but I will say that I do want one bad enough to save

 

I wish I could've got mine for those prices... my dealership said that at one point they were pretty cheap but the prices jumped pretty high pretty quickly... I paid about $60 at my dealership about 4 weeks ago...

 

4.56's and 31's is pretty steep. I ran that combo for a while and it was livable, but 4.56's go much better with 33" tires. Again it depends on where you drive. If it's almost all city driving then the 4.56's would be fine, and will offer great low speed crawl.
If you do much highway, then you may find the 4.56's/31's to be geared too low... I did out on the highway.
Personally, for daily driver and highway use, if sticking with 31's, I would go 4.10's.
If you think you may go to 33's, then I'd say go for the 4.56's.

BTW,
Generally 6 cylinder auto transmission XJ's came with 3.55 gearing, and manual transmission XJ's came with 3.07's.
If your rig is a 4cylinder, then, it could have anything from 4.10's up to the very rare 4.56's.