Tire Pressure Formula Validation

[GoPats]

Didn't find in FAQ and the 60 group has a guru that cranks them out by the hundreds with really fancy math.

Trying to apply science to figuring out what tire pressure I should run, I came upon this formula. Is it good as a ballpark?

GVWR divided by 4 divided by Max Load divided by Max cold PSI

I have a 1978 FJ40: tag says 4885
33x10.5x15 K02: Max Load 2600 (Per BFG Site)
Max cold PSI: 50

So, (4885/4)/(2600/50) = 23.49

Seems I see many folks (notionally and chalk measure) running 25 PSI.

Did I verify my bias, or is this actually correct?

 

[snaggletto]

Run whatever pressure you need to run to get the largest most even contact patch. Chalk up tires and drive a small distance in an empty parking lot. Adjust pressure to wear off the chalk on full width of the tire. There are to many variables to just go with numbers.

 

[lcwizard]

"Maximum Load" pressures and Maximum Psi are not always the same. You can have tires that achieve maximum load at 35 psi but
can safely run at 50. I've found that most 33" radials like 25-30 lbs for highway use on a 40 . A few more lbs for a 60. On the other hand
I've run for months at a time at 20psi on a 40 and 22 to 25 on my 60. If you have 38 Swamper TSLs you may not notice they're low until you get to under 10lbs by looking at them. Tire brands will vary in sidewall strength so I treat each tire a little differently.

 

[David70FJ40]

I have Pro Comp tires, 33x12.50x15 and 25 psi seems to work very well on road.

 

[65swb45]

Almost no Landcruiser weighs what the GVWR is, so that number is virtually irrelevant.

It’s all about the tire contact patch.

 

[bikersmurf]

I agree with others that the formula doesn’t make a lot of sense. Now that said, the answer is in line with the results I’ve got using the chalk test.

With my 4700 lb Fj40 on 33x10.50 BFG ATs mounted on 8” rims, I found 25 psi is slightly over inflated. I’ve chosen to run 25 for lighter steering (Armstrong steering) and hopefully slightly better economy.

 

[thorslc1977]

31 X 10.50 X 15 on 8" rims. Contact patch is best at 23 psi winter/ 21 psi summer ( Summers are hot here!)

 

[65swb45]

@fjl40 are you paying attention?

From 2015:

I had a run-in with Armstrong 30 years ago that apparently I have not shared online before.

I bought a set of 33x12.5s for my 76K5 at Price Club, the predecessor to Costco. Ran them at 32 cold, which was what the guys in my 4wd club were telling me was normal. After 12 months, all 4 tires had horrible scaling.

I drove to Price Club to ask them about it. The manager took one look at my lifted K5 and matter-of-factly pronounced I had alignment issues.

I was prepared for this.

I said "So let's suppose you're right about having an alignment issue. Let's also assume I'm a lazy MoFo and haven't bothered to rotate the tires for the year since I got them. That doesn't explain why the rear tires are as badly scaled as the front!"

He was not amused.

He called Armstrong, spoke to the factory rep for a couple of minutes, then said, loud enough for me to hear, "That's what I told him. You explain it to him." Then he put me on the phone with the rep.

The rep immediately asked me what the GVWR was for my truck. Since I knew this, I immediately replied "6200lbs." Startled, he said hang on a minute, then came back on the line and said "According to my spec book, that tire is not rated for a vehicle that weighs LESS than 7000 pounds."

I scanned my brain as fast as I could, and after about 5 seconds I said, "You know, I can't think of a 15" wheeled vehicle that weighs 7000lbs. So it sounds to me like you've designed a tire for which there is no correct application in order to avoid all warranty claims, and I wouldn't be surprised if this is the kind of thing that class action lawsuits are based on."

He said, "Let me speak to the store manager."

I handed the phone back to the manager, and watched as he slowly turned beet red. He hung up, turned to me and said "I am authorized to issue you a 100% non-prorated refund on the tires and let you keep them."

While it was comforting, it didn't 'solve' my problem. I went home and calculated the difference between the 7k reference and the ACTUAL weight of the truck as a percentage, then lowered the tire pressure from 32 cold by the same amount. Within 6 months, all the scaling was gone!

On all tires since then, I have used the contact-patch method.

 

[lcwizard]

What you need to remember about the load rating of tires is although an E rated tire will take more weight than a C that's only
when you run the tire pressure stated for that load. A "C" rated tire may attain a maximum load pressure at 35 psi where
an "E" rated tire may reach it's maximum load rating at 75 psi . The E will carry 25% more weight at that pressure but
once you drop the "E" tire to 25psi and the C to 25psi, the load rating is about the same if not the same for both tires.
Both follow a similar load curve until you pass the "C" rated max pressure. At that point the E , because of it's stronger carcass
can keep taking more pressure thus more load. Whatever load rating the tire has was calculated for a passenger vehicle.
You should subtract 10% on higher profile vehicles. The added side loading due to higher center of gravity in corners has to be
considered in the calculations unless you always drive on flat straight roads

 

[fjl40]

Yes sir, I've already aired down to a little below 25psi.

@fjl40 are you paying attention?

From 2015:

 

[jm599]

I run less in rears to decrease the highway “hop”.

 

[Downey]

Many guys with a wide foot print tire find them walking all over the road, lots of shimmy, and they find to get the tires to track better they run more air (less tire foot print on the pavement). Although this will in fact result in less tire wobble/shimmy, it will also result in the center of the tire tread wearing out sooner- - -trade-off time.