What PSI do you all run in your tires?

[chow]

I was just wondering what you all were running in your XJ, both on and off roads with size please
31X10.50X15
on 30psi
off 20psi

[blkTJ]

well, i don't have an xj but...

40psi on road.
9-12psi off.

[JeepGal]

I run around 37 on the road, and 10-13 off... but Im not sure how that applies to 31s

Tam

[Old Fart]

Time for the chalk test thread - I really ought to make a MS Word file so I can just cut and paste. This will tell you your optimum tire pressure for TREAD LIFE (which is not necessarily the optimum pressure for mileage, rain performance, etc)

ONROAD:

Pump up the tires to 35 psi COLD. Find a close parking lot where you can drive in a straight line for 50' or so. Steal some sidewalk chalk from your kids (or grandkids in my case)

Draw several chalk lines across the tread face of the tires. Drive forward _in a straight line_ for 30' or so. Get out and look at the lines. At 35 psi, you'll find that the line is worn out in the center, but not the sides. This indicates that you're OVERINFLATED.

Drop the pressure 3-5 psi, rechalk the tires. Repeat the above test until the chalk wears off EVENLY. Record this pressure, this is your OPTIMUM PRESSURE for tread life. Drive home, suck down a few beers, let the tires get cold. Record this pressure - this is what you inflate the tires to COLD to get optimum pressure (Tread Life).

Lots of things affect this, most noticibly tire case width vs. rim width. My 35x12.50's on 8" rims will run less pressure to get even tire wear than they would on 10" rims. Weight also affects the situation - a stock rig with no armor will put less pressure on the same tires than a fully armoured rig.

The number stamped on the side of the tire has ZERO to so with this - it's an indication of what the max pressure is to get the load rating out of the tire. For laughs, multiply the weight by 4 some time, and see what your rig would have to weigh to actually NEED that pressure.

Be aware that optimum pressure for tread life is usually less than what optimum about a zillion pounds of steel in the armour, bumpers and racks) needs only 24 pounds to achieve optimum tread life pressure - but that makes for Mister Toads Wild Ride going up and down the San Bernardinos. I'm willing to run them at 30 psi to get a better cornering situation - at the expense of tread life. Don't drive it every day to LAX, so it's an acceptable situation.

Off road, it's highly dependant on where/how you wheel. IMHO, for rocks - I'd start at 15 and go DOWN until you start debeading tires. Also depends a LOT on the tire - a bia ply has a thick sidewall, and needs less air to really flex. Rim width plays in here too - a 12.50 tire that will stay on an 8" rim all day at 10 psi will come off a 10" rim much easier (old desert racers used to run 33x12.50's on 7" rims)

[Dukes69]

Re: What PSI do you all run in your tires?

Originally posted by chow
I was just wondering what you all were running in your XJ, both on and off roads with size please
31X10.50X15
on 30psi
off 20psi

I had 33's on my XJ and ran 35 on the street and 12 offroad.
In my TJ I run 35 on the street and 9-10 offroad.

[Trailbst]

I agree with the chalk test, but disagree at the same time.

I recently had a talk with a friend. He brought up a good point. When talking about on the road tire pressure, wear is important, but you also have to think about what the tire is doing for you, and that is providing a good deal of lateral stability. Side to side flex. The chalk test make take too much air out of the tire to provide a good amount of lateral stability. Also keep in mind that everytime you slightly turn your tires, you are now using another part, i.e. the outer or inner, of your tires. When you go through a corner, you will lean on one part of the tire and not another, and this will cause the even wear that you are looking for.

The chalk test, and YES I was an advocate of it, is great if you plan on only driving in a straight line for the life of the tires.

In my opinion, you should be near or above 30 psi for on-road driving. Play with it. Watch for cupping, or uneven wear. Keep the alignment up to date. make sure that if you lifted it, you put on the appropriate control arms, since they will affect caster, which WILL affect your tire wear. I know that from my first XJ. Keep them balanced. Either through external weights, internal weights, BBs or a balancing disc. I know some with 38 Swampers who puts 2 pounds of BBs in them, and they have not worn unevenly since he put the BBs in a year ago.

You will get lots of advice, but only you can observe and correct what is happening on your Jeep.


Hope this helps.


Erik

[Trailbst]

Oh, forgot to give you my 2 cents about offroad.

Run 15 psi the next time your offroad. Try that, watch how much further you can go. Then when you are comfortable, try 12 psi. Any lower than that and you are looking at beadlock country.

I also noticed that you have 15x8. Don't go any wider than that, unless you plan on running a 14.5 wide tire. My buddy ran 35x12.5 on 15x7 rims on his XJ for years with no problems. The narrower rim helps the bead stay on. A cheap trick for XJs to help with the backspacing is to use the Ford Ranger rims.


Erik

[sarah]

Nah, 10-11 psi is ok... eh eh eh.

[CrawlBox]

32 on street, 11-13 on trail.

[goodtimes]

Old Fart....good post, but I want to add something to it. Not only do you need to remember the cold pressure (for when you check the air in the tires before you leave the garage), but also remember the hot pressure....for those times when you check the pressure after the tire warms up (like when stopping for gas....airing up after a trail, etc).

Trailbst, just in case you missed it, there is a disclaimer of sorts in Old Farts post about the chalk test being a good method to find the pressure that relates to optimal tread wear, not optimal handling. Which, btw, is why I run 33 pounds on the street instead of the 25 or so that the chalk test indicates. Off road, depending on what I am doing, anywhere from 9 - 17 psi (the tougher the trail, the lower I go---usually I am around 11 - 12).

[Trailbst]

Sorry Old Fart, I missed the disclaimer.



Erik

[Old Fart]

No big deal - I buried the fact that I run 30 in my 35x12.50's pretty deep in there

Off road "minimum" is probably as hotly "discussed" as on road. FWIW, the best method I've found is to actually run them until they debead, which will be affected by type of rim (bead seat area is different on an aluminum/steel rim), rim width, tire type - and most of all by type of trail.

As an example, I ran 33x10.50 TSL radials on 7" factory aluminum rims for a long time. Never debeaded them in the rocks at 11 psi - but they will come off the rims at that pressure on Cleghorn Ridge.

Run 14 psi with the 35x12.50 BFG MT's on 8" steel rims - will be going down to 10psi next trip to Big Bear to experiment. Having a Dana 60 in the back doesn't help (diff is about the size of Rhode Island).

[RUGD JP]

im running 30x9.50x15 MTR's. On road i run them at 30 psi, off-road(rocks) 10psi, (sand) 15-18psi.