On Highway Air Pressure in Oversized Tires

Tanktitan

Member
People often ask what air pressure they should run after they replace stock tires with oversized tires. Many assume that they always have to run the pressure listed on the door panel. I even heard one say that you should run what is on the tire sidewall. Most on this forum know that if you do either of these, you are going to jar the fillings out of your teeth.

I recently spoke with some Michelin/BF Goodrich (BFG is owned by Michelin) technical people and posed this question. The answer is that each tire is designed to support various weights at various PSI’s; vehicles are designed to support its own weight, plus other factors like G forces, potholes, hills and the like. Once the OEM tires are selected, the tire pressure on the door is set for the combination of that tire and the specific forces that vehicle is rated to experience.

Naturally, if you change tire sizes, the proper tire pressure changes for your vehicle. My 2016 JK came with BFG KM2 LT 255/75R 17 tires. FCA required BFG to support 2055 pounds per tire position. BFG called for 37psi to do so and Jeep put that on the door.

According to BFG, my 37” KM2’s should be run at 25psi to achieve a load rating of 2150 pounds. A friend contacted BFG about his 2014 JK, now running 35” KO2’s. They instructed him to run 30 psi.

To quote BFG’s email back to me: “If your vehicle has a different size tire than what was specified by your vehicle manufacturer, it is important that the tires be inflated to provide the same load capability of the original tire size at the manufacturer’s recommended pressure.

Based on the information provided your OE tire size LT 255/75 r17, PSI is 37 and the load is up to 2055 lbs. The new tire size 37x12.50x17, PSI is 25 and the load is up to 2150 lbs.

… if we can assist you further, please respond to this email or call us at 866-866-6605 (toll-free) between 8:00 a.m. and 8:00 p.m. EST Monday through Saturday.

We appreciate your business and thank you for choosing Michelin.”




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Tanktitan

Member
What if your vehicle weighs more than stock?

Good question. Based on what they say, 25psi in these tires support 8,600 pounds gross. Vehicles experience weights greater than scale weights, but this would seem to be an adequate safety factor for on highway even with our extra weight.


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OverlanderJK

Resident Smartass
What the fuck? You seriously wasted time calling bfg to ask what tire pressure to run?


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Noble Woodsman

New member
What the fuck? You seriously wasted time calling bfg to ask what tire pressure to run?


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I am willing to bet this is a fairly common question they get. After all, how many questions can there really be about tires?


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Ddays

Hooked
25lbs sounds kind of low. I usually run about 30lbs with 37s on pavement.


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Depends on the sidewall rating too. What are yours? Both my 37" Cooper STT's and 35" Nitto Ridge Grapplers are E and I run 26 in both.
I'm curious how the 40" Cooper STT's will be because they're C rated.
 
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Tanktitan

Member
Depends on the sidewall rating too. What are yours? Both my 37" Cooper STT's and 35" Nitto Ridge Grapplers are E and I run 26 in both.
I'm curious how the 40" Cooper STT's will be because they're C rated.

My KM2’s are D rated. I usually ran about 30. I have chalked my tires at that psi and the edges were not wearing like the center. Airing up to 25 is going to go quicker now.


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If you go to a tire dealer they will tell you to use the manufacturer recommended tire pressure even though your putting larger tires on the Jeep. I assume it's a liability situation, cover your ass.
 

jdofmemi

Active Member
If you go to a tire dealer they will tell you to use the manufacturer recommended tire pressure even though your putting larger tires on the Jeep. I assume it's a liability situation, cover your ass.

That's because they can't trust employees to think. If the corporate rule is go to what's on the door jamb, no one gets sued or fired.
 

Ddays

Hooked
If you go to a tire dealer they will tell you to use the manufacturer recommended tire pressure even though your putting larger tires on the Jeep. I assume it's a liability situation, cover your ass.

That's because they can't trust employees to think. If the corporate rule is go to what's on the door jamb, no one gets sued or fired.

Had an oil change done a ways back and when I left it felt like I was driving Fred Flinstone's goddam car down the road. Asshole tech filled the tires up to 65 psi because that's what it said the max was on the sidewall :2thumbsup:
 

Arrcherr

Caught the Bug
Had an oil change done a ways back and when I left it felt like I was driving Fred Flinstone's goddam car down the road. Asshole tech filled the tires up to 65 psi because that's what it said the max was on the sidewall :2thumbsup:
I always forget to tell them not to touch the tires lol IMG_20200121_162803.jpg

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Tanktitan

Member
If you go to a tire dealer they will tell you to use the manufacturer recommended tire pressure even though your putting larger tires on the Jeep. I assume it's a liability situation, cover your ass.

I am sure you are right. The engineer at BFG said they do not let the public use their programs to calculate the specific PSI for that specific tire on that vehicle but said they are happy to do so if people call. (BFG phone # is posted above.) Unsaid was the reason, but liability factors into every move we make in this litigious world. I commented that they might sell more tires if the dealers knew this. They said they would pass it to marketing.


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