Another thread about tire pressure

[tomk62]

I thought I understood the chalk test and how to determine the correct tire pressure.

So now I am totally confused and have no idea how to find what the correct daily driver/on road tire pressure is for my MT BB 35" tires. Do I just use this formula posted above?

https://www.mickeythompsontires.com/light-truck-tires/baja-boss-a-t

Tire Max Load 3000 lbs
Tire Max Pressure 50 psi

Vehicle GVWR 5600 lbs
5600 / 4 = 1400 lbs

1400 lbs / 3000 lbs = 0.47

0.47 * 50 psi = 23.3 psi

I should be running at 23 psi? (or more specifically, 22.5 front and 25 rear?)

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[Jtphoto]

I thought I understood the chalk test and how to determine the correct tire pressure.

So now I am totally confused and have no idea how to find what the correct daily driver/on road tire pressure is for my MT BB 35" tires. Do I just use this formula posted above?

Use the formula to put you in the ball part then use the chalk test to confirm. I usually run a few PSI higher than the formula. At the end of the day believe your chalk test. It is where the rubber meets the road.

 

[tomk62]

Use the formula to put you in the ball part then use the chalk test to confirm. I usually run a few PSI higher than the formula. At the end of the day believe your chalk test. It is where the rubber meets the road.

OK I edited my post, did I calc that correctly?

 

[azjl#3]

pretty sure with all the history around Fords explorer and firestones, the manufacture engineers do a pretty good job figuring out best tire and pressure for the vehicle. BTW, by using the spec pressure, which seems hi but is not, I wore out two sets of KO2 with zero odd wear patterns. The one set of KO2 i set at 25psi because thats where the chalk test showed the acceptable tread contact, had scalloped/cupped edges at 15,000 miles

 

[Jtphoto]

OK I edited my post, did I calc that correctly?

Generally we use actual weight not GVW but it’s close enough. You got the idea. Now do your chalk test and you will find 25-26psi will be good.

 

[tomk62]

Generally we use actual weight not GVW but it’s close enough. You got the idea. Now do your chalk test and you will find 25-26psi will be good.

How does one find the actual weight? Weigh it?

That calculated psi is significantly lower than I expected, and way lower than what i see most people run at. I'm currently at 32 cold which is what i thought was "close enough" with the chalk test. I guess that's way off.

 

[Jtphoto]

pretty sure with all the history around Fords explorer and firestones, the manufacture engineers do a pretty good job figuring out best tire and pressure for the vehicle. BTW, by using the spec pressure, which seems hi but is not, I wore out two sets of KO2 with zero odd wear patterns. The one set of KO2 i set at 25psi because thats where the chalk test showed the acceptable tread contact, had scalloped/cupped edges at 15,000 miles

I disagree. Look at the PSi on the Rubicon door jam. They state tire pressure to be 37psi for both the 33” standard Rubicon and the Xtreme Recon 35” tire. They didn’t do any figuring there.

 

[Jtphoto]

How does one find the actual weight? Weigh it?

That calculated psi is significantly lower than I expected, and way lower than what i see most people run at. I'm currently at 32 cold.

You can go to a local truck scale or ballpark it. A Regular Rubicon is around 4500lbs an Xtreme Recon about 5000lbs.

 

[tomk62]

You can go to a local truck scale or ballpark it. A Regular Rubicon is around 4500lbs an Xtreme Recon about 5000lbs.

Thank you. I worry about these things!

 

[Dg1615]

And I’ll say it again read up on it. Yes Some tires are made to run on the crown but most aren’t.
If you run enough miles and tires you will find out for yourself. The bigger the tire you get the less pressure it requires because it has more air volume.

I bought my first Jeep in 1983 and have had dozens of other vehicles as well. I feel like I’ve “ run enough tires and miles” to know what I’m talking about.

 

[Medsker]

I usually start out here but then adjust to conditions. I have almost 60,000 miles on my KO2's with a little tread left running them between 26-30 depending on temperature and conditions.

https://tiresize.com/pressure-calculator/

 

[tomk62]

I thought I understood the chalk test and how to determine the correct tire pressure.

So now I am totally confused and have no idea how to find what the correct daily driver/on road tire pressure is for my MT BB 35" tires. Do I just use this formula posted above?

https://www.mickeythompsontires.com/light-truck-tires/baja-boss-a-t

Tire Max Load 3000 lbs
Tire Max Pressure 50 psi

Vehicle GVWR 5600 lbs
5600 / 4 = 1400 lbs

1400 lbs / 3000 lbs = 0.47

0.47 * 50 psi = 23.3 psi

I should be running at 23 psi? (or more specifically, 22.5 front and 25 rear?)

This comes out to 33 psi (which is essentially what I'm running now) and is a full 10 psi difference to the calculation above! ? ?‍

https://tiresize.com/pressure-calculator/

 

[Al G]

Tire Maximum Pressure is the maximum air pressure the tire can handle. To calculate the recommended tire pressure, you divide the axle weight by the tire maximum load, and then multiply that by the tire maximum pressure.
ie
- Jeep axle weight (weight on each tire) 5,000lbs /4=1,250lbs
- 35” tire max weight 3225lbs
- 35” tire max pressure 65psi

1250 / 3225 = 0.3875969

0.3875969 x 65 psi = 25.1937psi

If you have a RAM pickup that weighs 7000lbs it would look like this with the same tires.

7000 / 4 = 1750lbs
1750 / 3225 = 0.54263566
0.54263566 X 65psi = 35.2713 psi

Check this formula with the figures on your stock 255, 265, or 285 tires.

That's an oversimplified formula. If you look at inflation tables from the manufacturers very rarely (if ever) is psi versus weight-carrying capacity a linear relationship.

 

[Jtphoto]

This comes out to 33 psi (which is essentially what I'm running now) and is a full 10 psi difference to the calculation above! ? ?‍

https://tiresize.com/pressure-calculator/

In that calculator your comparing the vehicle manufacturers tire spec not the tire manufacturers spec. That doesn’t work for our 35 tire spec on the XR.

 

[Jtphoto]

That's an oversimplified formula. If you look at inflation tables from the manufacturers very rarely (if ever) is psi versus weight-carrying capacity a linear relationship.

Yes it is a simplied version of the formula but it puts you in the ballpark. but like any formula, it’s only as accurate as the number you plug into it.
The chalk test is still the best.

At the end of the day, you do you. Whatever makes you happy. You can spend a whole day reading and researching the subject and you will find tire inflation charts from all tire manufactures.

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