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Unread 06-19-2006, 08:54 AM   #1
JeepinYahoo
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What is the correct street tire pressure and off-road pressure?

I was just wondering what air pressure I should run on the street, I have 31" BFG MT's adn I want to optimize them on the street and on the trail. I am running somewhere around 30-35 psi on the street and havent tried airing down on the trail yet. I just got a tire guage and I want to do this right, any suggestions help.

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Unread 06-19-2006, 09:07 AM   #2
Dr. Marneaus
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read the numbers on the side of the tires...that'll give you road pressure
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Unread 06-19-2006, 09:14 AM   #3
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rocks? sand? forest? 31x10.5's? 15x8 wheel? it'll make a difference. on the beaches, i run 8-10 psi, in the trees, i run 12-15, on my stock 15x8 wheels.
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Unread 06-19-2006, 09:37 AM   #4
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Quote:
Originally Posted by Marneaus
read the numbers on the side of the tires...that'll give you road pressure
The number on the side of the tire states the max pressure, not what you should run them at. The tire pressure depends on the weight of the rig, tire load rating. There's a chalk method to determine good street pressure, and offroad air down as low as you can get away with without blowing the bead.
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Unread 06-19-2006, 09:52 AM   #5
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I usually run mine at 30psi on the street...even tread wear thus far at 9500 miles on them (32" Procomp M/T's) (rotated at 4000 miles)
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Unread 06-19-2006, 09:56 AM   #6
mdm
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Quote:
Originally Posted by Marneaus
read the numbers on the side of the tires...that'll give you road pressure
No it won't. The pressure given on the sidewall of a tire is for carrying the stated maximum load of the tire, far too much pressure for normal driving. It would ride like a rock and wear the center of the tread.

Do a chalk test. Mark across the tread, drive a short distance (straight - driveway/parking lot). Check the chalk. If it is just worn off in the center, lower the pressure and try again. Keep going until you have even wear across the face of the tire.

There is a math process that will do the same thing for you.
On the sidewall of the tire you will find the max load/pressure of the tire.

For an example I’ll use a tire rated at 2270 lbs @ 50 psi and a vehicle that weighs 4850 lbs.

Step 1 - Multiply the max load times 4. This is the maximum weight your four tires can safely carry.
2270 lbs * 4 = 9080 lbs

Step 2 - Divide the weight of the vehicle (placard on driver’s door jam) by the number you got in the first step. This is the percent of the max load you are actually carrying on the tires.
4850/9080 = 53%

Step 3 - Multiply the psi stated on the tire for max load by the percentage you got in step 2. This will give you an optimal pressure for you tire on the road.
50 psi * 53% = 27 psi

The chalk test and the math approach will usually give you very close answers.
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Unread 06-19-2006, 10:11 AM   #7
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The weight of your jeep plays a HUGE role in determining air pressure for your tires. For instance, my TJ weighs 3,700 lbs. I am running 31x10.5 trxus mt's which are light truck tires. These can handle way more than what the tj weighs so you do not need alot of air pressure. I have run all my 31" tires under 26 psi for best wear and road manners.

Back when i first started driving and bought my first tires, my dad taught me this tire pressure formula that he has been using for years with complete success:

Tire: 31x10.5R15 2200 lbs at 50 psi

Vehicle: 2k TJ 3700 lbs

# of tires x sidewall weight: 4 x 2200 = 8800
All four tires can support a total of 8800 lbs
Total Weight / Vehicle Weight: 8800 / 3700 = 2.4 (tires can support nearly twice the jeep's weight)

2.4 / max psi : 2.4 / 50 = 20.8 psi needed per tire based on vehicle weight and total weight that tires can support.
I normally raise the front 2 psi because of the additinal weight up there so 22 psi front 20 rear.

I have used this formula for the past 10 years (since i started driving) and have not experienced any abnormal tire wear. I've also tried the chalk method (it works) but doing the math is quicker
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Unread 06-19-2006, 10:20 AM   #8
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Quote:
Originally Posted by mdm
No it won't. The pressure given on the sidewall of a tire is for carrying the stated maximum load of the tire, far too much pressure for normal driving. It would ride like a rock and wear the center of the tread.

Do a chalk test. Mark across the tread, drive a short distance (straight - driveway/parking lot). Check the chalk. If it is just worn off in the center, lower the pressure and try again. Keep going until you have even wear across the face of the tire.

There is a math process that will do the same thing for you.
On the sidewall of the tire you will find the max load/pressure of the tire.

For an example I’ll use a tire rated at 2270 lbs @ 50 psi and a vehicle that weighs 4850 lbs.

Step 1 - Multiply the max load times 4. This is the maximum weight your four tires can safely carry.
2270 lbs * 4 = 9080 lbs

Step 2 - Divide the weight of the vehicle (placard on driver’s door jam) by the number you got in the first step. This is the percent of the max load you are actually carrying on the tires.
4850/9080 = 53%

Step 3 - Multiply the psi stated on the tire for max load by the percentage you got in step 2. This will give you an optimal pressure for you tire on the road.
50 psi * 53% = 27 psi

The chalk test and the math approach will usually give you very close answers.
doh...you beat me to it. Looks like my math formula is spreading.
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Unread 06-19-2006, 10:23 AM   #9
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the sticker on the drivers side door frame tells me to run 33psi front and rear.

i have since changed tire size, but still run around 30-32 psi on the street. i have not done the chalk line yet, but they feel alright on the road. offroad i air down to about half that, 12-15 psi. i would go lower, but 12 is enough sidewall flex for me and i dont want to break a bead.
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Unread 06-19-2006, 11:41 AM   #10
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Quote:
Originally Posted by mattlock1983
the sticker on the drivers side door frame tells me to run 33psi front and rear.

i have since changed tire size, but still run around 30-32 psi on the street.
The manual says to run 29 psi in the 30" tire. Generally the larger the tire the less air pressure it needs to support the same weight. Think about bicycle tires. A thin racing tire requires 70+ psi while a "balloon" tire only needs 30-35 psi. The bike with the balloon tires will usually weigh far more than the racer and the guy riding the racer is usually a scrawny fellow too. So the thin racing tire is carrying less weight and needs far more air pressure than the balloon tire.
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Unread 06-19-2006, 11:42 AM   #11
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Quote:
Originally Posted by Muddeprived
Looks like my math formula is spreading.
Do I owe you a royalty?
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Unread 06-19-2006, 11:57 AM   #12
MeloYelo
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I run about 12 psi off road and 29-30 on road. 33x10.5 R15

How low have people been able to go before they break a bead? In CO there are rocks than sand and I'm curious how low you go.
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Unread 06-19-2006, 12:13 PM   #13
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On road I usually run 27-28 front and 26-27 rear with my 33x10.50s on 8" rims. It's probably a little high, but I usually run 16 off road.

Melo, how wide are your wheels?

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Unread 06-19-2006, 12:21 PM   #14
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I've done the chalk test a few times and I run 32psi on the street. I run 8psi front and 7psi rear when I'm offroad.
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Unread 06-19-2006, 12:47 PM   #15
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Quote:
Originally Posted by mdm
Do I owe you a royalty?
Yeah, as a matter of fact, just hand over the jeep

Jk, glad it's helping fellow wheelers out there
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