Is my tire pressure too high?

The eurometric has 1 loadindex step higher because calculated in maxload for 36 psi. The P- metric for 35 psi.
But in fact the same tire and if loadcapacity calculated for 29 psi it will give the same within some roundings.
Nowadays both are calculated with same formula, so that cant give the difference.
Before 2006 the P-tires where calculated with an even worse formula then that for diagonal tires. The European formula is used for decades here for all kind of tires, from standard load eurometric up to
trucktires.

For who it interests
Basic formula
Loadcapacity = maxload x (P/Pr)^X
P is pressure
Pr is reference-pressure ( that 35 or 36 psi).
X is power, like power 2 is squaire, and ^0.5 is root. Power ^1 is lineair calculation.
On sciencetific calculators is a button X^y for that.

Eur power for decades X= ^0.8 and since 2006 also US P-tires.
US power upto 2006 for P-tires ^0.5 (root).
US power for LT tires ^0.7.
US for trucktires also ^0.7, but nowadays often ^0.82. Higher power leads to lower loadcapacity for the pressure.

I think that is because to low pressure for trucktires, if lead to tirefailure, is sooner corrected, because large fleetowners can also afford expensive lawyers, but dont need to because they wont buy tires anymore from the tiremaker, if to much tirefailure.

The individual RV or car owner can be fooled by giving them the idea that it is their own foult.
In every country you see often the research information that 50% or more rides on to low pressure.

And often it is our own foult, but also often to low reserve calculated in the recomended pressure, especially RV and older vehicles.

 

At least OP will never have to inflate his tires ever again

 

I will say...you are taking some liberties with your math.

 

Your door says 32? I could have sworn mine said 30.

 

28 port, 58 starboard side, smart.

 

Different models, have different tires(sizes and load ratings)...they all have a PSI to attain a 2,149-2,172 lb inflated load capacity from euro-metric and P-metric tires.

He probably has an SR; OEM tire is a P-metric P245/75/16/109 @ 32 psi = 2,172 lbs.

You probably have a Offroad/PRO; OEM tire is a euro-metric 265/70/16/112 @ 30 psi = 2,149 lbs.

 

I run 28psi cold in my 285s. You must feel everything haha

 

Then I checked both lists, and the P-tires list is an old list, calculated with the old power 0.5.
The Eurometric with the 0. 8 power.

Example:
Load-index 94 second line
Maxload 1477lbs AT 35 psi

Loadcapacity 26 psi/ 1279 lbs
1477 x ( 26/35)^0.5 = 1273 lbs
I think the difference is because first calculated in kg and maby even in bar or kPa, and then rounded and conversed to psi and lbs and again rounded.

Edit: LI 94 is maxload 670 kg
670 x ( 26/35)^0.5 = 577. 46kg rounded to 580kg = 1279 lbs so spot on
So first in kg and rounded up even, and conversed to lbs.

Then LI 94 in the eurometric list.
1477 lbs AT 36 psi.
26psi /1135 lbs
1477lbs x ( 26/36)^0.8=1138.45 lbs
Rounded down to 5 lbs is 1135 lbs.

Second Edit:
670kg x ( 26/36)^0.8= 516. 47kg, rounded down to 515 kg is 1135lbs.
Have not checked it, but all the lbs in european list will convert to 5 kg steps.


So yust let me make a list for you.
I now use an even safer formula, based on an article of an American IR J. C. Daws.
Had often contact by mail with him and he gave me much usefull info in time, but unfortunately died febr 2017. A wise and open minded person.
That formula comes close to lineair calculation.
And in my list I give standard that 90% of calculated loadcapacity, and only round to 1 lbs at the end.

 

You are correct. I stand corrected and this is what is in my 21 Off-road




However when using the PSI calculator for the size the OP was discussing it does provide a limitation for minimum pressure of 35 psi...

 

How about you post the resources you are obtaining your info from?

 

End 2007 I got hold of the official european calculation and system for pressure for a certain load on tire, and went running with it. From page 14 vv of designersguide 2007 of ETRTO. Thats the reverse of the one I gave here.

and was trying to add a pdf of mr Daws, but wont work, will try later.

 

Here the pdf of IR J. C. Daws.
In that about the different used powers.
Also the about a calculation wich I explain as a zero pressure load, and of the rest lineair calculation.

And at the end advice if the old powerformula want to be used, for Personscar tires ^0.82 to use.
And I checked trucktireslists to be made with that power. Mr Daws adviced ^0.95 for those, wich would lead to even lower loadcapacity for the pressure, or higher pressure for same load.

 

the tire manufacturer actually tells you what psi is recommended :\

that said my stockers I ran 30 despite the door jam because I noticed rapid middle tread wear with near no outer tread wear at 35

watch the tire wear and adjust accordingly *shrug

 

thought mine said 35