Stop using "one-size-fits-all" tire pressures - a science based guide to the right tire pressure

[parb]

I wrote this guide after seeing a bunch of nonsensical statements made in offroad videos on YouTube.
For the best ride and traction, your pressure must be calculated based on vehicle weight and tire volume, not just what's printed on your door jamb.

Why it matters:

• Pneumatic Springs: Correct PSI allows tires to absorb high-frequency energy, saving your suspension and making the ride MUCH better
• Load vs. Volume: A 35" tire needs less dense air than a 33" to carry the same weight -sounds counter intuitive but it is true
• Precision: I’ve crunched the load-inflation math so you don’t have to.

I’ve included a cheat sheet table for common sizes (34s, 35s, etc.) and specific settings for highway, rock, sand, and snow.

Check out the full guide and the cheat sheet here: The Science of Tire Pressure

 

[Dokatd]

I wrote this guide after seeing a bunch of nonsensical statements made in offroad videos on YouTube.
For the best ride and traction, your pressure must be calculated based on vehicle weight and tire volume, not just what's printed on your door jamb.

Why it matters:

• Pneumatic Springs: Correct PSI allows tires to absorb high-frequency energy, saving your suspension and making the ride MUCH better
• Load vs. Volume: A 35" tire needs less dense air than a 33" to carry the same weight -sounds counter intuitive but it is true
• Precision: I’ve crunched the load-inflation math so you don’t have to.

I’ve included a cheat sheet table for common sizes (34s, 35s, etc.) and specific settings for highway, rock, sand, and snow.

Check out the full guide and the cheat sheet here: The Science of Tire Pressure

Good education or reminder!! This has bugged me for decades. The worst is the guy who airs up to max pressure of the tire.

I use chalk or even sometimes spray paint to get as close as possible to best possible street wear.

Sometimes the manufacturer will publish a rolling radius target. That's helpful too.

 

[parb]

I should mention that you need to adjust the pressure with the seasons. The tables are fixed and based on weight but you lose or gain 1 psi for every 10 degrees F change. This do vary a but based on how you drive but i would just adjust with the seasons for let's say every 20 pr 30 degrees temperature change

 

[Dokatd]

I should mention that you need to adjust the pressure with the seasons. The tables are fixed and based on weight but you lose or gain 1 psi for every 10 degrees F change. This do vary a but based on how you drive but i would just adjust with the seasons for let's say every 20 pr 30 degrees temperature change

If you don't mention it then the Grenadier TPMS will certainly do it. So frustrating here in Texas since we have such huge daily swings.

 

[parb]

Yeah, i think those on high altitude or desert conditions with cold nights and hot days are having a bit of a bother. Its a bit unfortunate that they made the temp change from set point to alert a fixed interval, i think it's too narrow.

I also find it really annoying that off-road mode doesn't mute the tpms pressure alert. I'm pretty much always aired down when I'm in off-road mode.

 

[Dokatd]

That's another item, off-road. You lay out pressures for off-road situations. I certainly understand the militant attitude for this and I have CTIS trucks that follow the same sort of pressures, but rocks or more specifically rock crawling requires much lower pressures. I'm not suggesting hard core rock crawling, but even more tame places like Moab will be vastly better at about 12psi rather than something upwards of 20psi. You start to get into comfort and tire protection zones depending on conditions. Tire protection can change pressures up or down.

I guess what I am saying is that once you get into certain rocky or even rooted terrain things become a little less scientific. And I hate seeing people beat themselves up on the trail with double the pressure they need.

But sub 10-12psi you will start to need bead locks for sure.

 

[parb]

I typically run 18-20 psi on rocks. I have 7,500 lbs Grenadier though.
I could go down to 15. At 12 i would want beadlocks.

 

[Tazzieman]

Keep going! You can do it!

 

[Eric]

Keep going! You can do it!
View attachment 7917561

Except not funny for my 3.5T trailer, that runs at 95psi.

 

[Tazzieman]

Except not funny for my 3.5T trailer, that runs at 95psi.

Wth?

 

[Eric]

Wth?

195/60 R12. Highway only, work trailer

 

[parb]

195/60 R12. Highway only, work trailer

not the most common size to find, especially if you're up in load index 108 for that kind of load. I don't think you can even buy that tire in the US.

 

[MileHigh]

Related to the temp and pressure thing, but something that I ran across last year. Highway driving with 285/70R19s KO3s, a 70-80 degree day, but at 7-10,000 ft altitude, highway driving, probably 75-85mph.. The tires were new and they had been filled to the door plat pressure- 35+ish? I ended up getting pretty high temps, maybe even a high tire temp warning. I'm guessing that the temp would be an issue mainly in highway driving, on hotter days.

What is 'too hot' for a tire like ours? Is there a specific tire inflation target for that? I was loaded, but not over loaded. Two people plus gear.

 

[Rok_Dr]

I wrote this guide after seeing a bunch of nonsensical statements made in offroad videos on YouTube.
For the best ride and traction, your pressure must be calculated based on vehicle weight and tire volume, not just what's printed on your door jamb.

Why it matters:

• Pneumatic Springs: Correct PSI allows tires to absorb high-frequency energy, saving your suspension and making the ride MUCH better
• Load vs. Volume: A 35" tire needs less dense air than a 33" to carry the same weight -sounds counter intuitive but it is true
• Precision: I’ve crunched the load-inflation math so you don’t have to.

I’ve included a cheat sheet table for common sizes (34s, 35s, etc.) and specific settings for highway, rock, sand, and snow.

Check out the full guide and the cheat sheet here: The Science of Tire Pressure

Nice one @parb. Thank you again for your science based insights.

One question would there be any changes to your formulas for Metric weights and pressures (kg and kPa) and if so what? I could work it out but I'm being christmas lazy

Cheers Steve

 

[MileHigh]

Would a 285/70R17 need a lot of tweaking from the 285/75R17? I think the 285/70 is a pretty propular unlifted option.

Also, this is only looking at width, any thought is the area due to greater diameter, but skinny tires?

 

[parb]

Would a 285/70R17 need a lot of tweaking from the 285/75R17? I think the 285/70 is a pretty propular unlifted option.

Also, this is only looking at width, any thought is the area due to greater diameter, but skinny tires?

The Physics: Contact Area = Vehicle Weight ÷ PSI.
If you keep the PSI the same, a 30" and a 40" tire have the exact same square inches of contact on the road.

The Shape: While the area stays the same, the shape changes. Taller tires give you a longer patch (better climbing); wider tires give you a wider patch (better flotation/off camber stability).

The Real Trick: Bigger, especially both wider and taller tires have more air volume, allowing you to run lower pressure safely.
Lowering the PSI is what actually increases the contact patch size, not just the rubber size. And increasing both dimensions gets you that ideal, an oval shaped patch.

What i did: I run 34s and as wide as i can on the Grenadier (nearly maxed out without rubbing on a stock vehicle).
The extra weight hurts highway MPG and acceleration, but the higher increase in footprint when aired down makes the trade-off worth it.

 

[parb]

Nice one @parb. Thank you again for your science based insights.

One question would there be any changes to your formulas for Metric weights and pressures (kg and kPa) and if so what? I could work it out but I'm being christmas lazy

Cheers Steve

Nah, it's all the same. Do you want me to generate a cheat sheet in metric?
I generally only compute in metric and convert to freedom units afterwards. I think physics is so much easier when done in metric.

 

[Clark Kent]

Nah, it's all the same. Do you want me to generate a cheat sheet in metric?
I generally only compute in metric and convert to freedom units afterwards. I think physics is so much easier when done in metric.

It's good work. Thank you for your contribution @parb.
My one request after reading it was it would better serve the global Grenadier community if it contained metric and freedom units. Since that's now in discussion I'll add my vote please.

 

[parb]

It's good work. Thank you for your contribution @parb.
My one request after reading it was it would better serve the global Grenadier community if it contained metric and freedom units. Since that's now in discussion I'll add my vote please.

You got it. It's a bit late here, I'll try to do it tonight, otherwise I'll get it done tomorrow

 

[Rok_Dr]

Nah, it's all the same. Do you want me to generate a cheat sheet in metric?
I generally only compute in metric and convert to freedom units afterwards. I think physics is so much easier when done in metric.

I see @Clark Kent has got in ahead of me but Yes please

Steve