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What is proper PSI for 33" tires?

61141 Views 38 Replies 25 Participants Last post by  Captain Skip
New to Jeeps, it is 90% me driving (170lbs) by myself. On the weekend towing a 350lb jetski with probably 30 lbs of tongue weight...so minimal. I searched a couple of different ways and did not find any specific mentions in the forum.

bf goodwrich 33x12.50R15LT tires...

I don't have a manual so I am in the dark. Just had a nail removed and the tire guy put 36 in them. Certainly rides a bit smoother since he did. But I don't know if that is correct or not.

Thanks in advance.
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Discussion Starter · #4 ·
Cool, they seem like nice tires, right? Haven't priced them as they came on the Jeep and I just got it Friday. I won't need a set in quite a while.

Glad the tire guy got that right. It definitely drives smoother than when the were at 30.
 

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36 psi is way excessive for that big of a tire, 26 to 28 psi at the most is appropriate for a 33x12.50 tire supporting a vehicle with the weight of a Wrangler.

And 30 lbs. of tongue weight is too light for a 350 lb. jetski plus the weight of the trailer. The mininum tongue weight is 10% of the total combined weight of the trailer and jetski, maximum of 15%. So if your trailer weighs 300 lbs. & your jetski weighs 350 lbs., your tongue weight should be 65 lbs. minimum. Without sufficient tongue weight, trailer sway is the result.
 

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What kind of rims do you have? stock or aftermarket. 12.5" wide tire on 8" rim will cause the tread to balloon out. I have 33x10.50's on 7" wide rim on my XJ and the tread balloons a little. I was running around 28 psi in them to keep the tread wear even.
 

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Please listen to Jerry B.I'd never run over 28 psi for 33's myself.
 

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Discussion Starter · #9 ·
I will play with alternate psi settings.

As for tongue weight, I will have to check on what my trailer is at now. It is pretty darn light if you ask me. But I have never measured it and never had any problems with swaying.

Thanks for the input!
 

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Its different for every jeep, just how every jeep is different. 25 psi works great for my YJ (33X12.5's)and I tow a small utility trailer, just try different pressures until you find whats right.
 

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Got my 33s installed and went strait to the San Rafael swell for some wheeling. Got 4 miles off highway and had to reinstall my teeth, align my back and put all my tools back the box. Checked the pressure and found the knuckleheads at an unamed national chain put 45 pounds in them. Put them down to 20 for the weekend and back up to 28 back on the highway. Left them at 18 once for about 50 highway miles and wore more off the front tires than the previous 3000 miles combined. 26-28 seems about right.
 

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I run 24psi in my MTZ's 33's
I did the chalk method and found 25 psi perfect for Mickey Thompson MTZ on 8" wide wheels.
Why is it that Mickey Thompson Tires like the MTZ's take lower psi compared to other brand tires? to have be best results in chalk test :confused:

I also have the MTZ tires but in 35" (with 8" tires) they seems to have even wear pattern with 20-22psi for a daily driver.
If that too low for a DD?
 

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Like has been said, chalk Test it! I found my 33s needed to be at 26#
 

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with the size tire specified on this thread they have a max pressure of 35 psi at somewhere around 2200 lbs per tire so at 35 PSI 2 Tires can pretty much support the weight of the vehicle. chalk it and if your worried about more than tire wear add a little more air but that will result in reduced tread life and the center of the tires being mostly worn out.
 

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New to Jeeps, it is 90% me driving (170lbs) by myself. On the weekend towing a 350lb jetski with probably 30 lbs of tongue weight...so minimal. I searched a couple of different ways and did not find any specific mentions in the forum.

bf goodwrich 33x12.50R15LT tires...

I don't have a manual so I am in the dark. Just had a nail removed and the tire guy put 36 in them. Certainly rides a bit smoother since he did. But I don't know if that is correct or not.

Thanks in advance.
If you want to know, the chalk test is great for getting your contact patch on the ground evenly.

As for what PSI that happens at...it depends on a few things.

For example, different load ratings can mean that different tire pressures will be required to get the same result.

The letter alone, like "C", is not enough either...you need the pounds at what max psi off the sidewall, etc.

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Generally, the chalk test works out to roughly what the door jamb placard recommended for the stock tires, for the stock tires.

The car makers tend to specify that the tires' psi is high enough to support ~ 35% of the rigs total loaded weight (GVWR) at each corner.

IE: A 4,000 lb GVWR rig would have ~ 1,400 lb capacity in each tire.

A 5,400 lb GVWR rig would have ~ 1,890 lb capacity in each tire.

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To GET that capacity, you simply use a percentage of the max psi that correlates to it.

For our 5,400 lb GVWR Example -

Lets say we have a 33 x 12.5/15 BFG KM2 that can support 2,205 lb at 35 psi max.

We need it to support 1,890 lb.

1,890 lb is ~ 86% of 2,205 lb, so we use 86% of the max psi to get it.

That works out to about 30 lb.

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Now lets see what a 33 x 10.5/15 BFG KM2 works out to:

They support a max of 2,600 lb at a max of 50 psi.

1,890 ln is only ~ 73% of 2,600 lb, so we take 73% of the 50 psi max to get it....

...Which comes to about 36 psi to support the same weight.

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Differences in tire construction and capacities change what psi is appropriate.

In the above examples we have 2 different 33" BFG KM2's, one that has a 50 psi max, and one that has a 35 psi max.

The 35 psi max only needed ~ 30 psi to support the SAME WEIGHT as the 50 psi max tire...but the 50 psi max tire, in this example, needed 36 psi to support that same weight.

(The above technique is not exact, as load capacity is not completely linear depending again, upon the specific tire, but its typically pretty close. The tire makers have charts that give the actual load capacities at different psi, which are helpful if you don't like math....and typically are within a psi or so)

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So there's no one universal psi that always works.

Its TIRE dependent.

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As for the chalk...that's RIG dependent.

The GVWR use is a basis for OEM recommendations. The manuals also typically recommend HIGHER than the placard psi for long range high speed driving, etc.

(OEM recommendations are for a soft, forgiving ride, and heavy lawyer friendly under steer in emergency maneuvers - not for performance.)

When you chalk test, its YOUR rig, and your tires, and your tools on board, etc.

If you were to do the test again after loading up with more than normal, or less than normal, etc....you'd ALSO end up at a DIFFERENT PSI...as more weight will then mean you need more PSI to get the same footprint, etc.

And that's why the chalk test is SO useful.

:cheers2:
 
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