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Tire Pressure Calculation


This is a discussion on Tire Pressure Calculation within the Tires / Wheels forums, part of the Jeep Liberty - KJ (02 to 07) category!
Hi all, I've done some research regarding the way to determine the correct psi for my new tires. I recently ...

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Old 09-01-2017, 04:41 AM   #1 (permalink)
 
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Default Tire Pressure Calculation

Hi all,
I've done some research regarding the way to determine the correct psi for my new tires. I recently changed after a lift, from 235/70R16 to LT245/75R16.
I have read several threads of users asking what tire pressure to use for their new tires but never an answer saying WHY...
The only link i found which explains why and how to determine the correct pressure was the following pdf by TOYO:
https://toyo-arhxo0vh6d1oh9i0c.stack...s_20170203.pdf

Therefore i would like to share it with you all...
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2004 JEEP KJ Limited
OME lift
FRONT: 2927/90009, Teraflex Bump Stops, Strut Spacer Plate set 1/4" + 1/4" JBA Clevis Spacer (Clevis spacer only left side)
REAR: 2948/n132l, Bump Stops
JBA A-arm
245/75R16 Cooper STT Pro
PML Transmission Pan
Aluminum Alloy Radiator with in radiator transmission cooler

Last edited by nbas; 09-01-2017 at 04:43 AM.
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Old 09-01-2017, 09:09 AM   #2 (permalink)
 
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Easiest way for me is to do the chalk test. Just need 1 piece of sidewalk chalk.

Fill your tires up to 40psi and use the chalk to make a line across the treads about 2" to 3" thick on all 4 tires. Drive in a straight line without any turning for 100" and see how the chalk wears. If you wear away the whole line your pressures too low. If you wear away only the center your pressure is to high.

About a 1/4" inch of chalk left on both edges is where I keep mine. For me with 32x10.5 load range E cooper's that's 35ish psi
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Old 09-01-2017, 09:24 AM   #3 (permalink)
 
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Yes you can do it with the chalk test and have an indication whether or not you are correct. If you are not, you will have to do it again with a different pressure according to your findings.
With the Load Inflation table you have a scientific way of calculating the exact pressure needed according to the load you put.
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2004 JEEP KJ Limited
OME lift
FRONT: 2927/90009, Teraflex Bump Stops, Strut Spacer Plate set 1/4" + 1/4" JBA Clevis Spacer (Clevis spacer only left side)
REAR: 2948/n132l, Bump Stops
JBA A-arm
245/75R16 Cooper STT Pro
PML Transmission Pan
Aluminum Alloy Radiator with in radiator transmission cooler
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Old 09-01-2017, 05:20 PM   #4 (permalink)
 
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I adjust to what I feel. I have with any vehicle that doesn't or didn't have OEM size and model tires. If OEM I'll run factory recommended psi. I don't follow the chalk or scientifics.

With larger tires and 35 psi I feel mine rides too stiff. At 30 psi it feels too soft and mushy. At 33 psi it feels good to me. Therefore I run 33.
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Old 09-03-2017, 11:42 AM   #5 (permalink)
 
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If you read the whole document, you will see that you can revert the manufacturer's recommendations to your new tires also, whatever kind of tires you choose.
The correct psi according to it for my new tires is 36psi.
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2004 JEEP KJ Limited
OME lift
FRONT: 2927/90009, Teraflex Bump Stops, Strut Spacer Plate set 1/4" + 1/4" JBA Clevis Spacer (Clevis spacer only left side)
REAR: 2948/n132l, Bump Stops
JBA A-arm
245/75R16 Cooper STT Pro
PML Transmission Pan
Aluminum Alloy Radiator with in radiator transmission cooler
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Old 09-03-2017, 09:52 PM   #6 (permalink)
 
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Originally Posted by nbas View Post
Hi all,
I've done some research regarding the way to determine the correct psi for my new tires. I recently changed after a lift, from 235/70R16 to LT245/75R16.
I have read several threads of users asking what tire pressure to use for their new tires but never an answer saying WHY...
The only link i found which explains why and how to determine the correct pressure was the following pdf by TOYO:
https://toyo-arhxo0vh6d1oh9i0c.stack...s_20170203.pdf

Therefore i would like to share it with you all...
Doesn't work most of the time as rim width effects the air pressure needed which rarely is taken into consideration and you also need to know your vehicles exact weight.

I've seen tire pressure calculators that say I need to run 40psi in my 35" MT/R K's,can't since the max rated pressure for my 35" tires is 35psi.The correct pressure is 27psi.My 315/75R16's need to be at 35psi but are range load E and mounted on a 10" wide rim instead of a 8" wide rim like my 35x12.50x15 MT/R K's.If I mounted my MT/R K's on a 8" rim I would need the full 35 psi and if my 315/75's were mounted on a 8" rim they would only need 30psi.
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Old 09-04-2017, 03:45 AM   #7 (permalink)
 
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Doesn't work most of the time as rim width effects the air pressure needed which rarely is taken into consideration and you also need to know your vehicles exact weight.

I've seen tire pressure calculators that say I need to run 40psi in my 35" MT/R K's,can't since the max rated pressure for my 35" tires is 35psi.The correct pressure is 27psi.My 315/75R16's need to be at 35psi but are range load E and mounted on a 10" wide rim instead of a 8" wide rim like my 35x12.50x15 MT/R K's.If I mounted my MT/R K's on a 8" rim I would need the full 35 psi and if my 315/75's were mounted on a 8" rim they would only need 30psi.
Load and Inflation Tables take into consideration the exact weight. If you know the exact weight for each tire, it is easier to find the correct pressure provided you find the correct Load Inflation Table for your tire...

I do not know how thoose calculations were made.Tires have specifications regarding the max psi which are met in the Load and Inflation tables. In fact, all tire manufacturers have to comply to the specific Load & Inflation standards according to the tire they produce (if it is a P-metric (T.R.A.), ISO Metric (E.T.R.T.O.), LT-Metric (T.R.A.), Flotation (T.R.A.)). Therefore you can't go wrong if you follow the correct table, as this is the standard under which the manufacturer has created the tire...
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2004 JEEP KJ Limited
OME lift
FRONT: 2927/90009, Teraflex Bump Stops, Strut Spacer Plate set 1/4" + 1/4" JBA Clevis Spacer (Clevis spacer only left side)
REAR: 2948/n132l, Bump Stops
JBA A-arm
245/75R16 Cooper STT Pro
PML Transmission Pan
Aluminum Alloy Radiator with in radiator transmission cooler
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Old 11-03-2017, 03:56 PM   #8 (permalink)
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I guess, tire pressure for BF Goodrich 245x70x16 All Terrain tires should be 26 PSI, since the load for one tire is less than 1841 lbs...
According to the chart, is that correct?
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Old 11-03-2017, 04:06 PM   #9 (permalink)
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Originally Posted by JEEPMAN2004 View Post
I guess, tire pressure for BF Goodrich 245x70x16 All Terrain tires should be 26 PSI, since the load for one tire is less than 1841 lbs...
According to the chart, is that correct?
26 would be too low
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Old 11-06-2017, 04:53 AM   #10 (permalink)
 
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For 1841lbs on each tire, since it is an LT245/70R16 tire, if you check the TRA LT inflation table, you should apply 42psi, as 40psi correspond to 1780lbs and 45psi correspond to 1935lbs.
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2004 JEEP KJ Limited
OME lift
FRONT: 2927/90009, Teraflex Bump Stops, Strut Spacer Plate set 1/4" + 1/4" JBA Clevis Spacer (Clevis spacer only left side)
REAR: 2948/n132l, Bump Stops
JBA A-arm
245/75R16 Cooper STT Pro
PML Transmission Pan
Aluminum Alloy Radiator with in radiator transmission cooler
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