Tire pressure question

I just had a set of P265/75R16 put on my 2003 DC TRD 4WD. (Went up +1). The door sticker says stock tire size of P265/70R16 should be inflated to 26psi. What psi should these new ones be?

 

26

Then do a chalk test. If you need more air, go up 1 lb per trial, till the test is passed.

https://www.4wheelparts.com/tire-wheel-package-guide/tire-pressure-checker.aspx

 

Thanks CB. WalMart inflated them to 40 psi!!!
I thought they rode a little rough. LOL

 

The chalk test seems VERY tedious and a bit more precise than I need. Would you recommend 26 psi and just "roll" with it? If anything, should I go up or down a little from 26 psi? The original tire size load range was 111. These are 114.

 

Chalk test is worth it. You'll get more miles and even wear from your new tires.

 

26 is low, fine if your cruising fsr's , I bet the chalk test is going to go around 34, what tires are they ? Load range ?

 

Kumho Road Venture AT51 114T

The placard on the door jamb says 26 psi for stock P265/70R16. These are only 1" taller and same width.

 

Maybe I'm all washed up on this but here is my train of thought.
GVWR 5100 lbs (I know, it's GROSS but let's go with it)
5100/4 (tires) = 1275 lbs per tire. Add 10% for good measure to bring it to 1402 lbs per tire.
My new tires are rated at 2601 lbs MAX at 51 psi MAX.
51/2601 = 0.0196 psi per lb
Using the 1402 lbs per tire...
1402 x 0.0196 = 27.5 psi per tire or 28 psi

 

You've spent more time noodling a formula, that you can't prove w/o a chalk test, that it would have taken you to do the first couple of passes at the chalk test.

Just mark it up before you go to work, and look when you get there. Adjust that evening, try again the next day.

You do need an accurate gauge and your own air supply to make this easier. I forget not everyone has those.

So. Everyone should have an accurate bleeder gauge. Period.

Then you could overfill, say 35, and start, but work backwards. Save you a trip for air every day if you have to go elsewhere to air up.

 

Actually didn't take that long. But I do see your point.

 

When you do a chalk test , start high and go down until even wear along the side, then stop.
If you go on until sides wear and middle not, you are on the low boder .
The 51 psi for a P-tire is the maximum allowed cold pressure, and not the pressure for wich the maximum load is calculated up to 160km/99m/h. For standard load P-tires this is AT 35 psi in American system ( 36 psi in European system) , and for XL/reinforced/Extraload P-tires this is AT 41 psi in American System ( 42 in European system) .
But P before the sises indicates that ist a American system tire.
This would give with the linear calculation that buckaroo gives , an even lower pressure, and american system does not go lower then 26 psi.

You cant yust devide GVWR by 4 , because GAWR rear is mostly a bit higher then front.
So look up your GAWR's ( gross axle weight rating) wich is maximum allowed by law.
Then add 10% for reserve, if you want to know pressure for fully loaded.
then do the linear calculation, wich is not that bad, gives a bit higher pressure then the official calculation wich I once got hold of and went running with.

The car maker yust goes from GAWR without reserve , and since 2006 the American TRA uses the same reasonably save calculation that European ETRTO used since decades.
So this system with 10 % reserve added will give next calculation when using linear calc.

5100/2= 2550 lbs front GAWR and rear GAWR +10% = 2805 lbs, but look this up on the same plate as your pressure advices.
Old tires Loadindex 111 = 2403 lbs maximum load , asuming Standard load AT 35 psi up to 99m/h.

Front 2550lbs + 10%= 2805 lbs / 2 X 2403 lbs= 0,5838 X 35 psi = 20,43 psi rounded up 21 psi.
This is the lowest European ETRTO allows ( USA 26).

Rear 2805 lbs + 10% = 3085 lbs / 2x 2403= 0,6419 X 35 psi = 22.47 psi rounded up 23 psi.

And this is for fully loaded ( and not overloaded) up to 99 m/h .
And for the old tires with loadindex 111.

You can do your own calculation for the new tires with Loadindex 114 /2601 lbs maxload, wich will even come to lower pressure.

But if you have offroad tires with large profile blocks , that cover a part of sidewall, this makes the sidewall lesser space to flex, so more heatproduction so you then better lower the Loadindex by 8 steps or 20% from maximum load.

I suspect the Bridgestone tires on the Ford Explorer , from the Ford/Firestone affaire, to be that kind of tire, and so for that the 26 psi advice gave to much heatproduction, and blowing rear tires , wich mostly courced roll over accidents, wich killed more then 100 people around the milenium chanche.

Greatings from a "Pigheaded Dutch Selfdeclared Tirepressure-specialist"
Story began when I got hold of the official calculation from ETRTO , and went running with it around 2008.

 

Googled a picture of the Kumho and it has mildly the profile blocks that cover a part of sidewall.
So I would derate it with 10 to 20% wich is 4 to 8 LI steps.
The 111 loadindex of old is 3 LI steps lower so about 7.5% derating.
so the lineair calc given in my last post , will do for the new tires.
Round it up a bit more, wich means that 26 psi is more then enaugh, so the 40 psi filled should be expected some discomfort and bad gripp.

Tire pressure advice is to give the tire a deflection for the speed driven, so not any part of rubber of tire goes above a sertain temp at wich it hardens and crackes by next bendings of that rubber by the deflections.
Once the structure of rubber is damaged , its ireversible, so when pressure highened up after that , still every bending of it tears the damaged part a bit further loose , and after a while a blowing tire , with the missery that goes with it.

Realise that at speed of 50m/h the wheel /tire makes about 10 cycles a second, so every segment of tire deflects and flexes back that 10 times a second, wich produces heat, wich gives the tire inside a sertain temp and the rubber also .

The tiremakers( organisation) has decided the deflection the tire may have for 99m/h . and calculated the maximum load for the AT-pressure.
The ideal formula to laws of nature, calculates for lower pressure the loadcapacity so the deflection stays the same.
This is as it is given in pressure/loadcapacity lists, so loadcapacity for pressure.
You can also turn it around and calculate pressure for a sertain load on tire.

So new feed for those who interests it.

 

But I love numbers and formulas...

 

I'm going to do the chalk test and see where that puts me as compared to the calculations of mine and jadatis.

 

When you do the chalk test, realise that a radial tire stays with its whole widht on the ground within a large range of deflection ( read pressure for the same load).
This deflection is speed related. So if you have the right deflection for a low speed say 60m/h , the chalk wears even.
But if you then ride 80m/h the tire might get to hot at places, so it damages.
The chalk test then gives if you go on lowering until sides wear and middle not, still to your idea a good pressure, but at higher speed the tire still damages by overheating.

So that is why I wrote that you have to begin at to high pressure wich will give more wear in middle then sides, then stop at the first pressure at wich the chalk wears evenly at sides and middle.

So filled at 40 psi , like you had first, will give to my idea a centre wear of the chalk . Think you can savely begin at 28 psi and lower by steps of 2 psi. Otherwise it will take weeks before you determined it all.

And bucaroo if you like formula's, sent me a mail at my hotmail.com adress with same username as here ( so jadatis) and I can sent you links and the copy of the European ETRTO formula, together with even longer story then here.
( like this to prefent spamm, you can combine the adress)