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Old 07-21-2010, 11:46 AM   #1
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Question Tire Pressure

I have a 2003 Jeep Wrangler and I just bought a new set of Kumho 31x10.50x15 All Terrain Tires. After I picked my Jeep up from the tire shop, it rode like a wagon from the wild west.I checked tire pressure and they were all around 45 psi.I lowered them all to around 38 psi and the ride was a bit better. My question is, with this size tire, what psi can I run that will have the best ride without causing the tires to wear prematurely?

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Old 07-21-2010, 12:26 PM   #2
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You should do a chalk test but I run 25 psi in my 30x9.5's.

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Old 07-21-2010, 12:32 PM   #3
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I'm running 27psi in my 31's. Nice smooth ride and no uneven tire wear.
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Old 07-21-2010, 12:38 PM   #4
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For 31" tires, I'd run 29 to 31 psi. Pretty close to 29 unloaded, 31 for when your Jeep is fully loaded. Those guys at the tire store were idiots for inflating them to 45 psi, they must be among the unschooled who think the correct tire pressure to run is what is molded onto the tire's sidewall. The pressure on the sidewall is only the tire's max safe pressure, not what should be run on the street. The correct tire pressure varies by the weight of the vehicle the tire is carrying so there is no one-single tire pressure that could ever be correct for all the various vehicle weights any particular tire might be carrying.
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Old 07-21-2010, 12:43 PM   #5
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Quote:
Originally Posted by jdenny View Post
I have a 2003 Jeep Wrangler and I just bought a new set of Kumho 31x10.50x15 All Terrain Tires. After I picked my Jeep up from the tire shop, it rode like a wagon from the wild west.I checked tire pressure and they were all around 45 psi.I lowered them all to around 38 psi and the ride was a bit better. My question is, with this size tire, what psi can I run that will have the best ride without causing the tires to wear prematurely?
When I used to run 31" MTs, my street pressure was maintained at 30 psi. I did not observe any unusual or premature wear at that pressure.
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Old 07-21-2010, 12:45 PM   #6
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Old 07-21-2010, 12:50 PM   #7
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35" tires...... 25 psi on the street
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Old 07-21-2010, 01:07 PM   #8
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I have 31 inch tires which are BFG's and I run 27 to 28 on the street.
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Old 07-21-2010, 01:12 PM   #9
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A good rule of thumb when you chalking the tires is to chalk it at the valve so you know where to look when you stop.
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Old 07-21-2010, 01:50 PM   #10
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Do the chalk test, but i bet you'll find that right about 28 is where you'll want to be...
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Old 07-21-2010, 01:50 PM   #11
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The chalk test is great at detecting overinflated tires but it also causes a lot of underinflated tires when not used with a little common sense. Some try to get the entire line to wear away 100% evenly during the test and keep lowering the pressure until the very edge is gone too which causes the tire to be underinflated.
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Old 07-21-2010, 02:13 PM   #12
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Wow!

Wow, I didn't expect to receive so many replies so quickly! Thank you all for your input. I am going to go home today and do a chalk test on my tires. Thanks again...

John
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Old 07-21-2010, 03:04 PM   #13
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question

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Originally Posted by 4Jeepn View Post
I checked the link, the straight line I got...short distance? 100 ft. or block or keep stopping and checking to see how much line is left. I know I am nit picking but I have an iquiring mind!
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Old 07-21-2010, 05:40 PM   #14
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I always liked 27PSI in my 31s and 32s. 35s ran great at 21 PSI, and my 37s liked 18PSI. My 39s don't see street very often(only in Moab), but they like 11-13PSI in the rocks.
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Old 07-21-2010, 05:41 PM   #15
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You just have to roll over it a few times. You can do the same thing by rolling thru water or dirt or mud, just look at the tires and see where the tread is touching. Like Jerry has mentioned numerous times, you don't want the entire tread to contact, but the majority of it. If you air them down enough for edge to edge contact they are under inflated.
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Old 07-21-2010, 09:08 PM   #16
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After reading some of these posts,I decided to try it on my crappy stock Goodyear GSA 30X9.50's.was running them @32 p.s.i.,went down to 26 p.s.i.-TREMENDOUS difference in ride quality.I never would have thought of this-thanks,all!The wife will not be happy with me buying new tires until I need them-can't wait for some real tires!
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Old 11-14-2013, 08:52 PM   #17
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One thought comes to mind on the chalk test and tire inflation. The load, or weight on the tires will vary from front to rear because the most weight is up front, unless you are towing or have some heavy weight in the back. Therefore, front tires will need more pressure than rear, then adjust for adding weight.
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Old 11-14-2013, 09:43 PM   #18
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You will find that not only does your Jeep ride a lot better with properly inflated tires, but it will also drive and handle better.
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Old 11-14-2013, 10:19 PM   #19
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Also pay close attention to what was said in post #11, the chalk and water "tests" are responsible for a lot of well intentioned but inexperienced Jeepers grossly understating their tires. For example, that 26 psi the person in post #16 above came up with grossly underinflated his 30" tires.
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Old 11-15-2013, 07:34 AM   #20
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...that 26 psi the person in post #16 above came up with grossly underinflated his 30" tires.
so...what's the correct answer for 30x9.5 tires? (on, say...a 98 tj 2.5/5speed )
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Old 11-15-2013, 08:16 AM   #21
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so...what's the correct answer for 30x9.5 tires? (on, say...a 98 tj 2.5/5speed )
I ran 26 psi in my 30's and they wore down evenly and use the same psi in my 31's.
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Old 11-15-2013, 09:23 AM   #22
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so...what's the correct answer for 30x9.5 tires? (on, say...a 98 tj 2.5/5speed )
That's a factory size tire where the factory has recommended at the correct tire pressure both in your owner's manual and on the door jamb sticker. 31-32 psi is usually a good pressure for that small of a tire when carrying a vehicle with the weight of a Wrangler, I'd just look in the owner's manual or doorjamb sticker to verify it. The 26 psi mentioned above is more appropriate for a significantly larger tire like you'd use for a lightly loaded 33x12.50 or more heavily loaded 35x12.50.
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Old 11-16-2013, 08:18 AM   #23
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That's a factory size tire where the factory has recommended at the correct tire pressure both in your owner's manual and on the door jamb sticker. 31-32 psi is usually a good pressure for that small of a tire when carrying a vehicle with the weight of a Wrangler, I'd just look in the owner's manual or doorjamb sticker to verify it. The 26 psi mentioned above is more appropriate for a significantly larger tire like you'd use for a lightly loaded 33x12.50 or more heavily loaded 35x12.50.

The door placard only lists for p205--33psi.
The manual has 2 different numbers. In the "tires" section inside the manual, it lists the 30x9.5 as 29psi; There's a quick-reference on the back cover that lists them as 33psi.
so...one of those, I guess.

I'll do the chalk test--how far do you have to roll to get the results?
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Old 11-16-2013, 08:48 AM   #24
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Again, I would not trust the chalk test by itself... by itself, without knowing what is reasonable for a specific tire size for a particular vehicle, the chalk test will just cause a tire to be under inflated. For a 30x9.50 tire when supporting a Wrangler, I'd use 29 around town with the vehicle carrying just 1-2 passengers. For highway use when the Jeep is loaded with gear/passengers, I'd raise it up to 32-33 psi max.

General guidelines on a tire's air pressure... One, there is no ideal PSI for a particular tire & tire size, that depends entirely on what weight that tire is carrying. The same exact tire and tire size will have different correct tire sizes which depend solely on the weight of vehicle it is carrying. Two, the correct tire pressure with any given vehicle weight is also dependent on the size of tire. For a Wrangler, 30 psi would be fine for a 30x9.50 tire but you'd only want 24-26 psi with a 35x12.50 Goodyear Wrangler MT/R when supporting the same weight. Generally speaking, the bigger the tire the lower the air pressure & visa-versa.

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