Tire pressure

[vantagetes]

My friend thinks I'm retarded because he's running 30psi in his 35"s on road, but I had to go down to 18psi to get full tread contact on my 33"s. I bumped up by 2psi in the front so now I'm running 20-18psi. He figures I'm gonna roll a sidewall and ruin my tires but everything looks right when it's sitting level. Minor bulge in sidewall, back is making full contact and front is still missing around 3/4" of wear on the outside (I figured should run a little higher in the front as the tire will get more force while cornering/braking).

Does this look right to you or am I out to lunch?

Back (18psi):


Front (20psi):


Back again:

[FiEND]

looks like mine.  but mine look about the same from 10 psi to 30 psi cause the sidewall is so thick/solid it don't change much.

i can run mine at 18-20 for a good feeling ride without much concern at all.  but i get better fuel economy at 25

the tires are built for weight.  the less weight, the less air they require to sit properly.

[vantagetes]

They came at between 32 and 36.5 (uninstalled) from the tire place (which was odd as the max load rating listed on the sidewall was 35psi...). I wasn't getting any wear on the outside lugs on the front at all.

I dropped to 25psi but could look underneath and SEE the gap at the sides still so I used a flattened cardboard box from an oil filter as a feeler gauge and just kept dropping the pressure until I couldn't slide it freely under the sides anymore (that was 18). Then I bumped the front up a little to account for braking/hard cornering and put a paint line across the tread. After test driving down the street the paint was worn off everywhere except the last 3/4" or so on the sides in the front. Had a friend do tight circles to see if I got mega bulge and everything looks good so I guess I'll call it done.

I'll probably come up a few lbs to up the fuel economy, maybe 22 and 20, see how that sits but man the difference in ride quality is awesome. Read: Doesn't ride like a T82 tank anymore.

[w squared]

If you did the paint test and like the results, you're probably good to go. Don't listen to your friend.

I'm with fiend in the "I run E-rated tires on my 4000 pound Jeep  " club. If a tire is made to put on a one ton, and you put it on your Jeep, you're gonna want to air down to get even tire wear and a ride that does not make your kidneys bleed. Based on your posts, you've got it figured out.

[FiEND]

they seat the beads at 40 or so lbs.  then let air out afterwards, just not enough i suppose.  you will get used to the harder ride at 25 and 20 will start feeling a bit spongy after they break in a bit.

mine also get flat spots when they sit for a week or more if they are 20psi or less.  take a while of bumpy bumpy bumpy thump thump to feel right again.  especially in the winter takes a while.

[JackstandJohnny]

I'll probably come up a few lbs to up the fuel economy, maybe 22 and 20, see how that sits but man the difference in ride quality is awesome. Read: Doesn't ride like a T82 tank anymore.

i run my tires at 12 usually on the trail to avoid popping beads
but they barely bulge.

i actually run at about 18-19 psi on the street. this is where the contact patch is even distributed over the tire tread.  its not bad. i also run at about 20 psi for some better mileage  

i've seen it a few times, guys have had tires badly 'cuppedd' because they wear in the centre, but 'swear' they ran the tires at the correct psi rated for the tire..........

in short, my jeep and my truck need much different psi ratings.................... so you have to play with it to find the best.

[Spinalguy]

Chalk me up for another guy that has run a jeep on street at the 20/18 club.
i don't think in the 7 sets of 33-37 tires i have run have ever seen above 26

[vantagetes]

they seat the beads at 40 or so lbs.  then let air out afterwards, just not enough i suppose.  you will get used to the harder ride at 25 and 20 will start feeling a bit spongy after they break in a bit.

mine also get flat spots when they sit for a week or more if they are 20psi or less.  take a while of bumpy bumpy bumpy thump thump to feel right again.  especially in the winter takes a while.

Good info, there's only around 250km on the tires so far I ran whatever was in them when they came from the tire place for the first little bit until I was finished going over the lift kit so I imagine they will soften up a bit more. I'll have to keep an eye (or ear rather) out for the flat spots and try to stay above that line. I guess if I run slightly higher the center will wear down eventually to give full contact (I joke)

[Zombie]

just for some technical stuff, I will check my books at work and see what they say, these are 35x12.5r15's?
what was the stock size and pressures??

I can see what each stock size tire will carry at a certain pressure and compare weights and find what pressures will hold the same weights. or if you actually know what the jeep weighs front and back, that would help.

with a buds jeep, stock was a 215/75r15 at 33 and with 33's to hold the same weight, it said something like 24-26 psi(can't remember), so in the low 20's sounds close

I like my 31's at about 28-30 on the xj, not too rough, but not sloppy either.

[vantagetes]

215/75/R15 @ 33 psi. And yeah 33x12.50R15. No idea what it weighs but it's a 2.5L so pretty much nothing. When I get the LoD tire carrier/bumper/rack/jerry can shizzle and my front winch bumper made up and winch mounted I'll have up come up a bit I imagine. Plus some tools and such, should soften up the suspension a bit too.

[Zombie]

I will try to have a look today and maybe play with the weight on the tag of the xj to give you an idea based on that.

[Zombie]

well the charts don't go as low as I had wanted them to, but to hold the same weight with the 33 as the stock tire does at 33 psi, you would have to run them at about 24 psi. now that is still holding about 1700 lbs per tire, which is no where close to what it actually has to be, so to hold the weight for highway speeds you could run them at about the 18-20 mark, this may be a little soft, but holds what it likely weighs(used my jeep with 1500 lbs front and 1700 lbs rear, which if maxed out, likely normally closer to 1300 front and 1100 or so in the rear, as a guess anyway)

[vantagetes]

Plus your jeep weighs a bit more as you've got a 4.0, four doors, heavier t-case, bumpers and a winch. Mines two door, 2.5 with nothing. Once I get my wheeling box stocked up, bumpers, winch, tire carrier, jerry cans etc I'll try them up at 24psi and see how it sits.

That chart is pretty handy for getting you a ballpark figure to go to and then make minor adjustments from there. Thanks Zombie!

[Rubi03 jef]

im the worse for  re airing up .. i can go months driving on 8 psi and besides not feeling speed bumps and loosing some milage no biggy .. probably not the best for the side walls and it doesnt help i drive with no front sway bar either

but if i do air up its 18- 20 psi max never more  and im running 35x12.5r15 mickey thompson mtz's

im sure if you were running a toyo open counrty you can drive those stiff F$#kers at 10 psi all the time and not be worried

i guess it come down to what you feel safe drivng with and if you car about wearing out your tires faster or balding out the centers because your at 36 psi and only 2 inchs of your tire is contacted with the road

[vantagetes]

My hoe operator has a "factory lift" aka dealer lift. I'm sure you've seen the JKs that all have the same lift/tires/rims. The max pressure on those procomps is like 60psi so hes running them at 50!!! I gave him crap but he's already down to 50% or so tread on the inside so it's gonna be near impossible to set his pressure right.