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-   -   Tire Pressure question (http://www.wranglerforum.com/f282/tire-pressure-question-110750.html)

GeePers 09-02-2011 02:10 PM

Tire Pressure question
 
Just put a set of Firestone Destination A/T's on my TJ. They are 33 x12.50x 15s. The primary useage is on the street or being towed behind our RV. When traveling we use the jeep on forest roads and are planning a trip to eastern Utah later this month. Anyone have suggestions for tire pressure for street use.
I realize that I need to air-down for sand, rough trails etc but the guys that installed the tires really had no suggestions and put 35psi in them. From watching the tires they seem to be slightly lifted on the edges but that may be my imagination.

NHrubicon 09-02-2011 02:16 PM

I'd personally run that size tire at 28-30# pressure for street driving. I am running 33x12.5 Cepek Crushers on my 03 rubicon. Offroad in the woods maybe 16# or so, I think once I went as low as 12# but the Crushers are so good on grabbing I'll stay with 16#. Running 28# on the street the tires are all worn about the same amount-no feathering or cupping.

Jerry Bransford 09-02-2011 02:37 PM

Quote:

Originally Posted by GeePers (Post 1534062)
Just put a set of Firestone Destination A/T's on my TJ. They are 33 x12.50x 15s. The primary useage is on the street or being towed behind our RV. When traveling we use the jeep on forest roads and are planning a trip to eastern Utah later this month. Anyone have suggestions for tire pressure for street use.
I realize that I need to air-down for sand, rough trails etc but the guys that installed the tires really had no suggestions and put 35psi in them. From watching the tires they seem to be slightly lifted on the edges but that may be my imagination.

35 psi was crazy-high, they probably just read what was on the sidewall which is never the correct air pressure to use.

For a 33x12.50x15 holding up a vehicle with the weight of a Wrangler, 26 around town and 28 on the highway when loaded is appropriate. :)

DeFazio 09-02-2011 02:44 PM

i have the recommended 50# psi in my 30s and they seem too low. any suggestions

jwm1986 09-02-2011 02:48 PM

Quote:

Originally Posted by DeFazio (Post 1534156)
i have the recommended 50# psi in my 30s and they seem too low. any suggestions

New tires and reading about the dangers of that much pressure in a tire that was not designed for it.

JeepnJim 09-02-2011 02:51 PM

50 psi. Wow.

Jerry Bransford 09-02-2011 02:52 PM

Quote:

Originally Posted by DeFazio (Post 1534156)
i have the recommended 50# psi in my 30s and they seem too low. any suggestions

What "recommended" 50 psi? If you're talking about what is on the sidewall, that is not the recommended psi, that is only the tire's maximum safe air pressure which is never the correct air pressure to use.

For 30" tires, you should be running the pressure on your door jamb's sticker or what is shown in your owner's manual which is somewhere close to 31-33 psi at the most.

neverfastenough1 09-02-2011 03:04 PM

I run what the vehicle calls for on all my vehicles. Only time I change the pressure is to go down the track or to go offroading. PSI recommendations are per vehicle weight on that particular axle. I know there are over thinkers on here that are thinking, well there are more square inches in a larger tire, so you don't have to run the same pressure to hold the same weight. Maybe so, but the recommended pressure works fine for me. Best way to make sure you are running the correct pressure is to check your footprint.

InvertChaos 09-02-2011 03:31 PM

I'm having good luck with 28psi on my 33x12.50x15 tires on 15x8 wheels.

freeskier 09-02-2011 10:49 PM

Quote:

Originally Posted by neverfastenough1 (Post 1534222)
I run what the vehicle calls for on all my vehicles. Only time I change the pressure is to go down the track or to go offroading. PSI recommendations are per vehicle weight on that particular axle. I know there are over thinkers on here that are thinking, well there are more square inches in a larger tire, so you don't have to run the same pressure to hold the same weight. Maybe so, but the recommended pressure works fine for me. Best way to make sure you are running the correct pressure is to check your footprint.

What about the fact that Jeeps have very dynamic weights? We are constantly modifying them, adding or reducing weight? It's also a known fact that larger tires require less air pressure.

DeFazio 09-02-2011 11:06 PM

Thanks fellas

cavediverjc 09-03-2011 01:06 AM

Quote:

Originally Posted by freeskier

What about the fact that Jeeps have very dynamic weights? We are constantly modifying them, adding or reducing weight? It's also a known fact that larger tires require less air pressure.

100% true.

TJWenatchee 09-03-2011 03:51 AM

I run 28-29 on my 31"s.

banks316 09-03-2011 04:02 AM

Read the side of your tire see what the max weight at max psi. I run 36" swamper iroks there rated 2860lbs @ 30psi. My front end weighs 1700ish divide that in half you get 850lbs each front needs to support. So I run mine on the high side at 20psi street, 4psi off road.

bc3_Jeep 09-03-2011 02:17 PM

Quote:

Originally Posted by Jerry Bransford (Post 1534132)
35 psi was crazy-high, they probably just read what was on the sidewall which is never the correct air pressure to use.

For a 33x12.50x15 holding up a vehicle with the weight of a Wrangler, 26 around town and 28 on the highway when loaded is appropriate. :)


Yeah..... what would the MANUFACTURER know about the correct amount of air to put in THEIR product??????

Seriously.... unless you are willing to put up a few MILLION dollars for liability coverage.... you should probably quit telling people to violate the manufacturers' recommendations....

Jerry Bransford 09-03-2011 02:27 PM

Quote:

Originally Posted by bc3_Jeep (Post 1536221)
Yeah..... what would the MANUFACTURER know about the correct amount of air to put in THEIR product??????

Seriously.... unless you are willing to put up a few MILLION dollars for liability coverage.... you should probably quit telling people to violate the manufacturers' recommendations....

If you are talking about what the sidewall says, you need to learn a few things before accusing me of providing false information on correct tire air pressures.

1) The correct tire's air pressure depends on how much weight the tire is supporting. The heavier the vehicle's weight, the more air pressure required in a given size tire to properly support it. The same size tire will be used on many vehicles that all have different weights. For example, the 35" MT/R is used on vehicles from Wrangler TJs to military Humvees that can easily weigh double what a Wrangler TJ weighs. You would not use the same air pressure in a 35" MT/R to support a Wrangler TJ as you would a fully loaded military Humvee. So right there that should be a good indication the air pressure on the tire's sidewall is not going to be correct for all applications.

There is no single air pressure that could possibly be correct for all vehicle weights a particular size tire may be used on.

2) The air pressure molded onto a tire is really only there to show its maximum safe air pressure. Not its recommended air pressure for all possible vehicles that have widely varying weights. See #1 above. The sidewall pressure is only there to show what air pressure should be used if the tire is supporting its maximum safe weight. Since tires typically carry no more than about 50% of their safe maximum load, which is so they have an adequate safety margin, the air pressure on the sidewall would never be correct for a typical vehicle installation.

For further education on the subject since you are apparently unbelieving of my advice, do a little research on the Internet. One such site at http://www.discounttire.com/dtcs/infoSidewall.do verifies what the sidewall's air pressure is really indicating:

"Maximum Air Pressure - "300 kPa [44 psi]"
This indicates the maximum operating inflation pressure of the tire. It does not indicate the manufacturer's recommended inflation pressure, nor does it indicate the proper air pressure based on the vehicle the tire is mounted on. This category is also based on NHTSA standards.
"

It's amazing how few people know what the air pressure molded onto a tire's sidewall really means. Not even many tire jocks that work in tire stores know what it really means.

GoldenSahara00 09-03-2011 02:30 PM

The air pressure is also for HOT air, so when you put it in it needs to be LESS than what you want the tires to run at when warm by a few psi. The air is cool when you put it in the tire. :)

Jerry Bransford 09-03-2011 03:02 PM

I should have added this additional information from Discount Tires to my above #16 post just above the information I posted on the air pressure...

"Maximum Load Limit - "635 kg [1400 lbs] (for example)"
This indicates the tire's maximum load-carrying capabilities when the tire is inflated to its maximum inflation pressure, as indicated on the sidewall. Max load is based on standards set by the National Highway Traffic Safety Administration (NHTSA)"

That was just above the information I quoted above already and provides further clarification.

TLATEOTW 11-01-2011 06:44 PM

Quote:

Originally Posted by Jerry Bransford (Post 1536252)
If you are talking about what the sidewall says, you need to learn a few things before accusing me of providing false information on correct tire air pressures.

1) The correct tire's air pressure depends on how much weight the tire is supporting. The heavier the vehicle's weight, the more air pressure required in a given size tire to properly support it. The same size tire will be used on many vehicles that all have different weights. For example, the 35" MT/R is used on vehicles from Wrangler TJs to military Humvees that can easily weigh double what a Wrangler TJ weighs. You would not use the same air pressure in a 35" MT/R to support a Wrangler TJ as you would a fully loaded military Humvee. So right there that should be a good indication the air pressure on the tire's sidewall is not going to be correct for all applications.

There is no single air pressure that could possibly be correct for all vehicle weights a particular size tire may be used on.

2) The air pressure molded onto a tire is really only there to show its maximum safe air pressure. Not its recommended air pressure for all possible vehicles that have widely varying weights. See #1 above. The sidewall pressure is only there to show what air pressure should be used if the tire is supporting its maximum safe weight. Since tires typically carry no more than about 50% of their safe maximum load, which is so they have an adequate safety margin, the air pressure on the sidewall would never be correct for a typical vehicle installation.

For further education on the subject since you are apparently unbelieving of my advice, do a little research on the Internet. One such site at Reading the Tire's Sidewall - Discount Tire verifies what the sidewall's air pressure is really indicating:

"Maximum Air Pressure - "300 kPa [44 psi]"
This indicates the maximum operating inflation pressure of the tire. It does not indicate the manufacturer's recommended inflation pressure, nor does it indicate the proper air pressure based on the vehicle the tire is mounted on. This category is also based on NHTSA standards.
"

It's amazing how few people know what the air pressure molded onto a tire's sidewall really means. Not even many tire jocks that work in tire stores know what it really means.

hahaha i seriously recommend everyone to carefully read what jerry posts before replying to him............many were lost for being that careless :p

dabolt55 11-01-2011 08:14 PM

I am only guessing after reading posts, that maybe being at 30 psi would be ideal??

InvertChaos 11-01-2011 08:17 PM

Quote:

Originally Posted by dabolt55
I am only guessing after reading posts, that maybe being at 30 psi would be ideal??

Depends on your tire size

TJeepman 11-01-2011 09:38 PM

Some people run the "max" cold pressure stated on the sidewall of the tire. The tire is designed to take that pressure and more (considering the tire will heat up with driving) without failing. These tend to be people trying to get better fuel mileage and are prepared to put up with the rougher ride.
From Recommended tire pressure - Transport Canada
Quote:

The maximum tire pressure marked on the tire sidewall refers to the pressure required to carry the maximum load of the tire, and is generally not the same as the manufacturer's recommended tire pressure for your vehicle. To find the recommended tire pressure for your vehicle's tires, refer to the information label, which is usually located on the edge of the driver's door, or the door post or other conspicuous location.
Overinflation versus underinflation from Tire Tech Information - Air Pressure - Correct, Underinflated and Overinflated
Quote:

Disadvantages of Underinflation

An underinflated tire can't maintain its shape and becomes flatter than intended while in contact with the road. If a vehicle’s tires are underinflated by only 6 psi it could lead to tire failure. Additionally, the tire’s tread life could be reduced by as much as 25%. Lower inflation pressure will allow the tire to deflect (bend) more as it rolls. This will build up internal heat, increase rolling resistance and cause a reduction in fuel economy of up to 5%. You would experience a significant loss of steering precision and cornering stability. While 6 psi doesn’t seem excessively low, remember, it usually represents about 20% of the tire’s recommended pressure.

Disadvantages of Overinflation

An overinflated tire is stiff and unyielding and the size of its footprint in contact with the road is reduced. If a vehicle's tires are overinflated by 6 psi, they could be damaged more easily when running over potholes or debris in the road. Higher inflated tires cannot isolate road irregularities well, causing them to ride harsher. However, higher inflation pressures usually provide an improvement in steering response and cornering stability up to a point. This is why participants who use street tires in autocrosses, track events and road races run higher than normal inflation pressures. The pressure must be checked with a quality air gauge as the inflation pressure cannot be accurately estimated through visual inspection.
I find that tires, inflated to the psi stated on the door placard, tend to wear more on the edges and less in the center. After all, that pressure on the door placard is a compromise between "wear", "handling" and "comfort". Inflating by 4psi more will likely result in more even wear. If the center starts showing more wear, cut back the psi by a couple. Keep monitoring via tread depth gage, at every tire rotation, and adjust psi accordingly.
Very Important: Tire pressures should be checked/adjusted when tires are cold per what the Owner Manual for the vehicle says. A tire at 33 psi cold, could show 34 psi when the ambient temperature warms up by 10F. From: http://www.tirerack.com/tires/tirete....jsp?techid=73
Quote:

The rule of thumb is for every 10 Fahrenheit change in air temperature, your tire's inflation pressure will change by about 1 psi (up with higher temperatures and down with lower).


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