Proper tire pressure for non-stock spec tires? - JeepForum.com
 
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post #1 of 9 Old 01-01-2006, 05:11 PM Thread Starter
JohnnyWamp42
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Proper tire pressure for non-stock spec tires?

I recently got a pair of 235/75/15 BFG AT's, with a maximum tire pressure of 50 psi, which is what the tire shop inflated them to. I just read in Click and Clack, though, that you're not necessarily supposed to inflate to max pressure, and are supposed to inflate to the manufacturer specs. They said this only applied to original sized tires, though, and that for larger-sized tires, the proper tire pressure is more or less a guessing game. I was just wondering what pressure anybody with 235's has found works best, and of course what pressure works best for pavement, and what works best for trail.


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post #2 of 9 Old 01-01-2006, 05:21 PM
numchuck
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best to consult your owners manual . . . that will be a safe starting point for highway driving . . . if really needed - try decreasing street pressure by 1/3 for trails - but try not to go much below 18 psi .

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post #3 of 9 Old 01-01-2006, 05:40 PM
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the best way to determine the tire pressure you need to run is to look at your tire. take a piece of chalk and draw a line across the tread from side to side (across the width og the tire) then drive a short distance. get out and look at your chalk line. it SHOULD be completely worn off if you are at proper pressure. if just the center has worn off and you can clearly see the outer edges still, lower your pressure. if the outside edges have worn off and the center is still on, you are too low. increase your pressure.

My X-terrains say 35 PSI max, which I have found is too high. I run them at 28PSI on the street and its perfect

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post #4 of 9 Old 01-01-2006, 08:40 PM
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Since the tires are bigger than your stock tires and hold a heavier load, your best bet is to start with the pressures listed for your stock tires. They usually are on a sticker on the door jamb, glove box, or if all else fails check the owner's manual. That will put your pressure real close to what the engineers designed. If you do lots of highway driving, that pressure should be fine. If you are mostly a low speed town driver, you could drop a couple psi for a slightly smoother ride.

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post #5 of 9 Old 01-01-2006, 09:06 PM
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My guess is those are like 30" tires. I have 31" BFG A/T and run them in the high 20s you should stick to high 20s/low 30s.
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post #6 of 9 Old 01-01-2006, 10:23 PM
BigWaylon
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Quote:
Originally Posted by JohnnyWamp42
I recently got a pair of 235/75/15 BFG AT's, with a maximum tire pressure of 50 psi, which is what the tire shop inflated them to. I just read in Click and Clack, though, that you're not necessarily supposed to inflate to max pressure, and are supposed to inflate to the manufacturer specs. They said this only applied to original sized tires, though, and that for larger-sized tires, the proper tire pressure is more or less a guessing game. I was just wondering what pressure anybody with 235's has found works best, and of course what pressure works best for pavement, and what works best for trail.

the key is that's a MAX pressure...at a MAX weight rating...

according to this site:

http://www.bfgoodrichtires.com/asset...rain_ta_kd.pdf

the 235/75R15 is rated at 50 PSI @ 1985 lbs....which means you'd run 50 PSI if your vehicle weighed 7940 lbs...

figuring you probably weigh a little over half of that, you ought to be running a little over half of the 50...somewhere in the 28-30 PSI range is probably good...

but the chalk test will prove what that number actually is...

Greg

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post #7 of 9 Old 01-01-2006, 10:47 PM
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Quote:
Originally Posted by BigWaylon
the key is that's a MAX pressure...at a MAX weight rating...

according to this site:

http://www.bfgoodrichtires.com/asset...rain_ta_kd.pdf

the 235/75R15 is rated at 50 PSI @ 1985 lbs....which means you'd run 50 PSI if your vehicle weighed 7940 lbs...

figuring you probably weigh a little over half of that, you ought to be running a little over half of the 50...somewhere in the 28-30 PSI range is probably good...

but the chalk test will prove what that number actually is...

Greg
Well I run 31x10.5R15 and it says they are rated at 50 PSI @ 2250 lbs. 2250 lbs x 4 tires = 9000 lbs. My Jeep probably weighs around 3500-4000 lbs mean I should run mine at around 20 PSI? That seems a little low to me.
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post #8 of 9 Old 01-01-2006, 11:43 PM
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First, as mentioned, the air pressure molded onto a tire's sidewall is only its maximum allowed pressure which is only for when the tire is carrying its maximum allowed weight. Since few tires used on Jeeps are carrying more than half the tire's max allowable weight, most correct air pressures will be closer to around half that pressure.

A few methods to determine a good pressure involve either chalk or a business card. Draw a line across the tread with chalk and drive a short straight distance to see how the chalk wore off. If more chalk was removed in the center of the tread, the tire is overinflated. If more chalk was removed from the outside edges, the tire is underinflated.

You can also use a business card for checking the pressure. Park on a smooth flat surface like a concrete garage floor and see how far underneath the tread the card can be slid. You want to be able to easily slide the card in under the tread but not so much air pressure that the card can be inserted too far in, like over 1/4" or so.

Remember that generally speaking, air pressures need to be higher on smaller tires and lower on larger tires. My 35" tires do well on the street with around 26 psi which would be too low for a 30" tire.

And those tire pressures on the door jamb? Those are what is recommended when the vehicle is fully loaded with passengers, baggage, and full fuel. If you normally drive alone or with just 1-2 passengers, your tire pressure should be adjusted downwards accordingly.

And check your tire pressure when the tires are cold, not after they are warmed up or hot from driving. Hot air expands so it will be higher when it's hot. Tire pressures are meant to be checked and adjusted when the tires are cold.

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post #9 of 9 Old 01-02-2006, 10:32 AM
BigWaylon
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Quote:
Originally Posted by MCanavan
Well I run 31x10.5R15 and it says they are rated at 50 PSI @ 2250 lbs. 2250 lbs x 4 tires = 9000 lbs. My Jeep probably weighs around 3500-4000 lbs mean I should run mine at around 20 PSI? That seems a little low to me.

the number will always seem a little low...because of places like tire shops that almost always over-inflate tires...

the proper pressure for my Iroks is around 14PSI...since they're rated at max of 35PSI @ 2850 lbs...

do the math...then test it with the chalk test, or drive through a very light water puddle (as in just spray your driveway), or find a flat dirt road...

you'll see how the stuff hits the tread all the way to the outer edges...

now, the "right" pressure will give you the maximum grip and wear...but not the but fuel efficiency...keep that in mind...

if you inflate yours to 50 PSI...you will see better gas mileage as there will be less rolling resistance...

Greg

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