Did I do this chalk test correctly?

[emarrero]

First timer chalk testing here. 2021 JLUR, replaced factory BFG's with Baja Boss 35 x 10 (34.6" x 9.8"). They're `E` rated LT tires. Tire Rack shipped them to a Pep Boys for installation and they filled them to 37 lbs because, well, Pep Boys. I knew it was probably the wrong pressure and figured I'd chalk test to find the right one.

I settled on 30 lbs because anything lower felt like it might be unsafe on the highway so I'm turning to the experience of the group to validate (or not) my method & choice to not go lower.

I used the TPMS measurements since we all know that is widely accepted as the most accurate way of measuring tire pressure known to man, and it honks the horn

Here are the pics, open to all advice on this but I will definitely smile and scroll past "Don't use TPMS". Here are the pics:

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[azwjowner]

30 seems like a good choice to me. I think you could go slightly lower, like 28-29, as well. You are correct that for the chalk test, the very outer edge of the tread isn't supposed to touch the ground much during normal operation. So you're testing for even wearing off of the chalk almost all the way to the edge, but not quite.

I like to look at the tire pressure load charts, which define the minimum pressure required to safely carry a rated load (at max speed, worst case scenario). There's no 255/85/17 chart that goes below 35 psi because it's a LT-rated tire, but the flotation tire sizes have a 35x10.5R17 size listed which is very close -- just a half-inch wider: https://www.toyotires.com/media/pxcjubjs/application_of_load_inflation_tables_20200723.pdf (page 31)

For 35x10.5, the tire can carry 1,760 pounds @ 25 PSI. 35x11.5 is 1,830 pounds @ 25 PSI. So by extrapolation (tire capacity is roughly proportional to volume of air), 35x10 is about 1,700 pounds per tire @ 25 PSI. That tells us you can probably run 25 PSI safely, from a blowout/load perspective. That doesn't mean handling is ideal -- sometimes too low and turns are mushy, although that's usually people with very wide tires (35x12.5). Given all of that, plus experience (I run my 32x10 tires @ 32 PSI on a 2 door) that PSI decreases as tire volume increases, somewhere around 28-30 PSI would seem to make sense to me. I suspect I'd run them at 28, personally.

 

[Carolina Jeeper]

I have basically the same tire, just in the MT version. I'm down to 26 PSI and have no issues. If I go on long trips loaded down with more people and gear, I bump the pressure up to 30 psi.

You'll never get a full tread contact with these tires on factory width wheels. I've tried, and the tire never made enough contact on the outer edges to wear off the chalk fully. The lowest I was willing to go was 25 for highway speed testing and the results look no better. It's demishiing returns unless you deflate to a dangerously low air pressure.

Maybe someone with wider wheels will chime in to confirm what I think.

 

[jadmt]

I have been getting perfect tread wear for the last 40 years using the 10% rule. for me tire chalking on really heavy sidewalled tires ie e rated has never really worked on lighter vehicles like jeeps. even at 10psi with the chalk test it would show ok...we all have different methods but I have found if my psi stone cold rises 10% when fully warmed up I am golden. for ease of math if I am at 30psi cold and I go up to 33psi after 10 or 15 miles on the highway i am good to go. If it goes up more than 10% ie if 30 psi cold it goes up to 35psi or higher it probably too low to begin with and if it does not go up 10% ie if stone cold only goes up to 31-32psi it is too high to begin with.

 

[emarrero]

If I understand correctly, starting with cold tires, if your pressure rises more than 10% while driving, there's too much friction at the cold temp, right?

 

[jadmt]

If I understand correctly, starting with cold tires, if your pressure rises more than 10% while driving, there's too much friction at the cold temp, right?

Yes because they are too low to begin with

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