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Old 10-30-2012, 09:28 AM   #1
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Thoughts on tire air pressure

I put a new set of 31x10.5 Cooper Discoverer A/T3's on my TJ a couple months ago. The dealer put 35lbs in the tires when I bought them. When I would pull into my shop, I could tell the whole tread was not contacting the pavement. I lowered the pressure to 30lbs now and it still seems as though I'm not contacting the outer edges of the tread pattern. Is it ok to keep lowering the pressure or am I going to start running into problems on the road.
My jeep is a weekend driver and hunting/fishing rig, not a daily user. Tires are just too expensive not to care for them.

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Old 10-30-2012, 10:00 AM   #2
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30 psi is about right for a 31" tire. Keep in mind that with most tires the outer edge of the tread it is not normally supposed to be in contact with the road.

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Old 10-30-2012, 04:42 PM   #3
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I ran my 31s at 29 for 50,000 miles so far and they seem to be wearing evenly.

The chalk test is how I tested mine. Get some of that big sidewalk chalk (having a kid around we had plenty) and chalk up the tread of the tire then drive it a bit and look and see how the chalk is wearing off. When it comes off evenly you should be good.
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Old 10-30-2012, 04:43 PM   #4
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Yep I'm running 28 in my 33/12.50/15 will see how they wear and go from there.
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Old 10-30-2012, 04:51 PM   #5
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I lowered mine to 30psi from 35, theyre brand new 31x10.5x15 duratracs and its my DD,as advised by a member from here, rides smoother and should wear evenly, checked my spare and the tire shop filled it to 40psi, should I leave it as or lower it to 30psi, since its not mounted I would think it doesn't matter unless I need to use it. The spare is one of my old BFG TA KOs same size, keeping it until I need a rotation then ill get a new duratrac to have a matching spare rotated with my set.
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Old 10-30-2012, 06:26 PM   #6
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I run 28-29 in my 31s and have not had a wear issue. Your right tires are way too expensive not to care about.
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Old 10-30-2012, 06:27 PM   #7
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Quote:
Originally Posted by SouthTJ View Post
I ran my 31s at 29 for 50,000 miles so far and they seem to be wearing evenly.

The chalk test is how I tested mine. Get some of that big sidewalk chalk (having a kid around we had plenty) and chalk up the tread of the tire then drive it a bit and look and see how the chalk is wearing off. When it comes off evenly you should be good.
Caution that the chalk test has probably produced more underinflated tires than any other method as the inexperienced try to get the chalk off evenly all the way out to the edges of the tread.
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Old 10-31-2012, 09:08 AM   #8
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Thanks, I'm going to try chalking the tread on one. I'm also going to put the chalk on the concrete and see what kind of track I get.
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Old 10-31-2012, 01:48 PM   #9
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what psi do most people run with their 33's?
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Old 10-31-2012, 02:23 PM   #10
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lol those dealers and their 35psi's

30psi sounds right, just remember all jeeps have different weight due to mods etc

for 33's I have 24psi on mine and the end of the tread still doesn't touch but it runs fine and wear pretty even.
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Old 10-31-2012, 02:27 PM   #11
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I run my 33's at 30psi. Sometimes I drop them down to 28. Tires are wearing great.
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Old 10-31-2012, 02:45 PM   #12
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I run my 33's at 30psi. Sometimes I drop them down to 28. Tires are wearing great.
30 psi is excessive for a 33" tire. 26-28 is about right for a 33x12.5.
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Old 10-31-2012, 02:49 PM   #13
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Quote:
Originally Posted by Jerry Bransford
30 psi is excessive for a 33" tire. 26-28 is about right for a 33x12.5.
Ok that's good to know. I will keep them at 28.
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Old 10-31-2012, 04:55 PM   #14
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I have 31's, tried 28lbs for a while but seemed to mushy. I then tried 30 which I have stuck with. I have even tried 33 which is what is posted in the door jam but 30 seems the best ride.
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Old 10-31-2012, 06:11 PM   #15
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what about stock size 30x9.50 15 , I am running them at 30-31
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Old 10-31-2012, 08:01 PM   #16
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I have 31's, tried 28lbs for a while but seemed to mushy. I then tried 30 which I have stuck with. I have even tried 33 which is what is posted in the door jam but 30 seems the best ride.
The door jamb pressure is only for the size tire that the factory installed. Larger tires require less air pressure.

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Originally Posted by shanes04wrangler View Post
what about stock size 30x9.50 15 , I am running them at 30-31
I don't recall what the factory tire PSI is but 31-33 psi rings a bell. No matter, the correct air pressure for your factory size tires can be found on your door jamb and in your owners manual.
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Old 10-31-2012, 08:41 PM   #17
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the door jam says 33psi but from what i understand and was told is the psi in the door jam is more for mpg and less for comfort and tire wear that why i bumped it down to 30psi, its a lot smoother.
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Old 10-31-2012, 08:42 PM   #18
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the door jam says 33psi but from what i understand and was told is the psi in the door jam is more for mpg and less for comfort and tire wear that why i bumped it down to 30psi, its a lot smoother.
The high end of the given pressure range is for when the Jeep is fully loaded with passengers and gear, the lower pressure is for when it is lightly loaded.
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Old 10-31-2012, 08:44 PM   #19
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so you think i should bump it back up to at least 32psi ? what about slick conditions?
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Old 10-31-2012, 08:55 PM   #20
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so you think i should bump it back up to at least 32psi ? what about slick conditions?
If it's just you or maybe you and one passenger, you can run the lower pressure. The higher pressure is only for when the Jeep is maxed out with passengers and gear.

Another time to run the higher pressure is when the streets have water on them like during a rainstorm. A higher tire pressure decreases its footprint which means it is less likely to hydroplane on top of standing water on the street. Which is exactly why a water ski with its big surface area stays on top of the water better than a skinny snow ski would that would cut down through the water.
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Old 10-31-2012, 08:59 PM   #21
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ok i get it now

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