Tire pressure for 31s? Same width means same pressure?

[SSTJ]

I usually run my 30 x 9.5 tires at 29 psi on my stock-light 97 SE. That’s what the sticker in the door jamb tells me to do.

I’m going to mount some 31.5 x 9.6 tires (245/75/R17). Since they are about the same width, I figure there will be about the same amount of tire in contact with the ground at the same tire pressure. Right? So I should stick with about 29 psi?

Edit: Very helpful answer from @freedom_in_4low, right here. Also a handy tool from @RMETeeJay right here, though I can't believe the result. Also found a similar thread here.

Conclusion: I'll stick with 29psi for my 30s, because that's what the door jamb recommends. For the 245/75/17s, which are being mounted on JK Moabs (7.5" wide, I think), I'll start with 28 because they're bigger and have a stiffer sidewall. I may play with the chalk test (thanks @reddvltj), but it sounds like the real way to make sure you aren't abusing your tires is to watch how it wears down over time.

 

[Gollywomper]

Generally the larger the tire the less air that is needed. More square inches means less pounds per inch needed to hold a specific weight.

 

[LONGJP2]

The same air pressure may or may not give you the same contact patch and/or ride comfort.
Depends on the amount of sidewall flex.
No harm in starting at 29.

 

[Kid Mechanic]

Do not go more than 27.38psi OR less than 27.63psi. DO NOT FUCK THIS UP!

Someone from the committee will be double checking this in the near future, or later.

 

[Gollywomper]

Do not go more than 27.38psi OR less than 27.63psi. DO NOT FUCK THIS UP!

Someone from the committee will be double checking this in the near future, or later.

Those numbers are impossible……..

 

[freedom_in_4low]

I usually run my 30 x 9 50 tires at 29 psi on my stock-light 97 SE. That’s what the sticker in the door jam tells me to do.

I’m going to mount some 31.5 x 9.6 tires (245/75/R17). Since they are about the same width, I figure there will be about the same amount of tire in contact with the ground at the same tire pressure. Right? So I should stick with about 29 psi?

The door jamb sticker on my LJ also says 29.

Diameter changes it too. You wouldn't run 29 in a 35x9.5.

Maybe check the load ratings of each tire and see what you end up with if you reduce the pressure by the same percentage.

 

[SSTJ]

Diameter changes it too. You wouldn't run 29 in a 35x9.5.

This is what I'm interested in. I believe you, but why is that true? I realize that the difference between my 30s and my 31.5s is minimal, so I'm not losing sleep over this. Just want to understand.

 

[bucky]

If you are changing to metric tires you need to inflate them to 2 bar.

 

[SSTJ]

I just want to understand. Maybe @Jerry Bransford will enlighten me.

 

[srimes]

This is what I'm interested in. I believe you, but why is that true? I realize that the difference between my 30s and my 31.5s is minimal, so I'm not losing sleep over this. Just want to understand.

Wider tires make the contact patch bigger by making it wider, so less pressure is needed. Taller tires make the contact patch bigger by making it longer, so less pressure is needed.

 

[freedom_in_4low]

This is what I'm interested in. I believe you, but why is that true? I realize that the difference between my 30s and my 31.5s is minimal, so I'm not losing sleep over this. Just want to understand.

ok. I'm not a tire expert but this is the way it breaks down in my head. If somebody with more expertise sees something wrong with this thought process, please speak up.

So let's say we have a 4000 pound Jeep with perfect weight distribution...1000lb/corner, and a certain area (the contact patch) of the tire in contact with the ground. In that contact patch and based on the first law of physics, you have 1000 pounds pushing up on the outside of the tire and 1000 pounds pushing down on the inside of the tire (we'll neglect the support provided by the sidewalls since it's really not significant as can be seen by how flatly it squishes when the tire is flat).

So that 1000lb pushing down on the inside of the tire is air pressure spread out over the area of the contact patch. So the higher the pressure, the smaller the contact patch and vice versa.

Now where size comes in, is that size of the tire does not change this. So a 35x12.5 at 30 psi is going to have the same contact patch as a 30x9.5 at 30 psi. The problem is that the size of contact patch for one is not appropriate for the other. To make the 35 have the same contact patch as the 30 means it's underloaded/overinflated, riding hard, not wearing evenly and probably helping your gas mileage by an amount that matters to the EPA but a real human will never notice. To make the 30 have the same contact patch as the 35 means it's overloaded/underinflated, riding way too comfortably , wearing unevenly and to the extreme may overheat and blow out on the interstate.

To into examples, let's take two pairs of tires of the same model where one has the width as the variable and one has the diameter.

BFG KO2
31x10.5R15 2270@50psi
33x10.5R15 2600@50psi

33x12.5R15 2205@35psi
35x12.5R15 2535@35psi

The fact that in each case, the larger tire has a larger load rating at the same pressure demonstrates that the larger tire is intended to have a larger contact patch, and to maintain the appropriate contact patch for the tire at the same load, the larger tire would take a lower pressure.

 

[RMETeeJay]

I usually run my 30 x 9.5 tires at 29 psi on my stock-light 97 SE. That’s what the sticker in the door jam tells me to do.

I’m going to mount some 31.5 x 9.6 tires (245/75/R17). Since they are about the same width, I figure there will be about the same amount of tire in contact with the ground at the same tire pressure. Right? So I should stick with about 29 psi?

https://tiresize.com/pressure-calculator/ gives me 22PSI with your parameters. I’d run through it yourself to double check.

 

[SSTJ]

ok. I'm not a tire expert but this is the way it breaks down in my head. If somebody with more expertise sees something wrong with this thought process, please speak up.

So let's say we have a 4000 pound Jeep with perfect weight distribution...1000lb/corner, and a certain area (the contact patch) of the tire in contact with the ground. In that contact patch and based on the first law of physics, you have 1000 pounds pushing up on the outside of the tire and 1000 pounds pushing down on the inside of the tire (we'll neglect the support provided by the sidewalls since it's really not significant as can be seen by how flatly it squishes when the tire is flat).

So that 1000lb pushing down on the inside of the tire is air pressure spread out over the area of the contact patch. So the higher the pressure, the smaller the contact patch and vice versa.

Now where size comes in, is that size of the tire does not change this. So a 35x12.5 at 30 psi is going to have the same contact patch as a 30x9.5 at 30 psi. The problem is that the size of contact patch for one is not appropriate for the other. To make the 35 have the same contact patch as the 30 means it's underloaded/overinflated, riding hard, not wearing evenly and probably helping your gas mileage by an amount that matters to the EPA but a real human will never notice. To make the 30 have the same contact patch as the 35 means it's overloaded/underinflated, riding way too comfortably , wearing unevenly and to the extreme may overheat and blow out on the interstate.

To into examples, let's take two pairs of tires of the same model where one has the width as the variable and one has the diameter.

BFG KO2
31x10.5R15 2270@50psi
33x10.5R15 2600@50psi

33x12.5R15 2205@35psi
35x12.5R15 2535@35psi

The fact that in each case, the larger tire has a larger load rating at the same pressure demonstrates that the larger tire is intended to have a larger contact patch, and to maintain the appropriate contact patch for the tire at the same load, the larger tire would take a lower pressure.

Awesome, thank you.

 

[SSTJ]

https://tiresize.com/pressure-calculator/ gives me 22PSI with your parameters. I’d run through it yourself to double check.

Wow, super handy tool. Thanks for sharing.

But seriously? 22psi? That's a big drop for a slight increase in tire size.

I just ran through the online tool myself and got the same result. I like the tool, but I'm having a hard time believing/trusting it.

 

[freedom_in_4low]

Wow, super handy tool. Thanks for sharing.

But seriously? 22psi? That's a big drop for a slight increase in tire size.

I just ran through the online tool myself and got the same result. I like the tool, but I'm having a hard time believing/trusting it.

that's probably the load rating coming in. He used load range B (96) for your stock tire, and SL (112) for your new tire. I have no idea if the stock tires were really load range B but the results may change if you use C instead.

I agree though, 22 would weird me out even if I 100% believed all the numbers. I run 25 in 35x12.5 load C.

 

[srimes]

Wow, super handy tool. Thanks for sharing.

But seriously? 22psi? That's a big drop for a slight increase in tire size.

I just ran through the online tool myself and got the same result. I like the tool, but I'm having a hard time believing/trusting it.

22 is way low for the size. I run 35x12.5 and the calculator gives the right answer for that. Must be a load rating issue?

 

[Vtx531]

Nothing to do with contact patch. It has to do with air volume. 17" wheel has less air volume than a 15" wheel of the same size tire and would need higher psi for the load capacity part.

That being said, load carrying capacity is only one of many factors that go into the recommended tire pressure for a specific vehicle. The general rule is don't go lower than the door placard but you may need to go higher if the load capacity requires it.

Just a few things besides load carrying capacity that tire pressure affects- rollover tendency, stability, spring rate, hydroplaning, steering, ride quality, tire heat buildup, etc etc. The engineers take this all into to account in addition to just the load carrying capacity.

You will notice most all vehicles have a tire pressure of at least mid 30's even though that load capacity far exceeds what is needed and that is not by accident.

 

[Vtx531]

22 is way low for the size. I run 35x12.5 and the calculator gives the right answer for that. Must be a load rating issue?

View attachment 298047

Tire load/inflation tables dont go below 25 psi so there is no way to calculate it.

 

[SSTJ]

that's probably the load rating coming in. He used load range B (96) for your stock tire, and SL (112) for your new tire. I have no idea if the stock tires were really load range B but the results may change if you use C instead.

I agree though, 22 would weird me out even if I 100% believed all the numbers. I run 25 in 35x12.5 load C.

Yep, I went back and used C, and even then got the same results.

 

[Old_Crow]

For what it's worth, I went up to 31x10.50R15's on my Jeep from the factory 30x9.50's. I calculated the new tire pressure by starting at 29(door sticker pressure)and lowering it until I wore all the mold tits off the tread of the tire. When I got to 25 psi, all the tits, all the way to the edge were gone.
I just ran my Jeep specs through that calculator, and it told me the new pressure should be 25 psi. Seems like it's pretty close.