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Old 04-01-2021, 11:04 AM   #1
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chart for load capacity as function of pressure

I can't seem to find such a chart on any of the tire manufacturer sites, even though I am certain that they were required to develop during engineering and DOT approval process.


Within the choice of new tires for my RAM 1500, there are 44 psi max tires and 51 psi max tires, all of which are load index 115T.



Question 1: What is the load capacity for a 44 psi versus 51 psi when tire pressure is set at the truck placard's pressure of 40 psi? Which one is capable of handling more load?



Question 2: When having weight in the bed plus tongue weight of a 6600 lb trailer, am I better off using a 44 psi max tire inflated to 44, or a 51 psi max tire inflated to 51? Isn't rolling resistance a function of tire roundness and footprint?
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Old 04-01-2021, 10:52 PM   #2
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Re: chart for load capacity as function of pressure

hey timeking.. first, welcome to Automotive Forums..
now, hopefully, at least one (of hopefully many) answers on your tech question..
first, you are right; the tire manufacturer is required to meet, or exceed, certain specs upon designing their tires, and have charts.. but they are quite well hidden..
classictrucks.com in the late 90s did a complete article and rundown on how to read the side of every tire on the planet.. it is in their archive DIY sections; and if you cannot find, i have a copy that i can post, or email..
that said, onto portions of your question.. you mention RAM1500, the Dodge became the RAM1500 back in '94/96.. i do not know which year/model you have, but from the late 90s to the newest '21 RAM; tires (in make/model/size) have changed in design..
in the mid/late 90s, your rig was running 225/75/15s to 245/70/16s; and the brand new '21 base (Tradesman) can run from a 17-18", and the more elite models can have up to a 22" wheel/tire combo..
that said, depending upon diameter size, aspect ratio, tire hardness, and load index (and sidewall) can justify different psi ratings..
so, if your truck has on its placard says load index 115, that is about 2400lbs. that your single tire can carry safely; regardless of psi..
psi should be as close to max as possible to insure best overall mpg, and to assist the speed factor, which allows a tire to travel at 'X' speed w/o allowance for that 'squishy/spongy' rollout as you round a corner and inertia and weight bear down on the tire's sidewall, as the geometry of forced mass changed the amount vehicle weight shifts from center mass of tire attached to earth, and moves to outer rim areas..
so---

Q1- if both 44 & 51 psi tires are rated at 115 then amount of load/weight they can carry are same.. a 51 psi tire will get poorer mpg at 40psi than a 44psi tire..
a 51 psi tire will be able to handle more mass at speed than 44, as weight has to do with stationary downward gravity, mass is inertial property or 'matter' and directional movement..

Q2- yes, rolling resistance of 51psi on your rig with moving mass in bed and on tongue will be the best overall for mpg and to assist in removing "roll" or spongy cornering..
the 51 probably has a higher treadwear number, as a 250 would perform two and a half times better than a tire with a 100 rating.. and it might have a better traction or a better A/B/C grade, as its ability to absorb and dissipate heat will be better..

if you want more info: coker tires has a more complete physics course than i can type..
coker.com or 800-251-6336
sorry for being wordy... but i hope this helps..
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Old 04-02-2021, 06:41 AM   #3
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Re: chart for load capacity as function of pressure

Ah ...... Mmmmm ....... Not exactly.

Ya' see, the load tables (as they are called in the tire industry) are set by Tire Standardizing Organizations. I talk about them here: http://barrystiretech.com/tirestandardizingorgs.html

The load tables are the same regardless of who manufactures the tire. The quirk is that there are different tire standardizing organizations throughout the world and they have SLIGHTLY different answers to the question (Emphasis on the word" SLIGHLY!!"), so you have to be careful when comparing tires to understand which standard they are following.

44 psi vs 51 psi? That value is arbitrary: I talk about that here: http://barrystiretech.com/loadtables.html

The load tables are the same for both (given the same tire standard)

Yes, It's complicated.

But for practical purposes, the load carrying capacity is the same at 40 psi regardless of what the max pressure is and which tire standard they are following. Put another way: The laws of physics decide how tires behave and the tire load charts are different ways of describing that.. And because the approaches are different, there will be SLIGHLY different answers, but the tire doesn't care. It's going to follow the laws of physics!

HOWEVER, there is a difference between SL (Standard Load) and XL (Extra Load) tires and they have different load carrying capacities - BUT - they have the same load carrying capacity at the same pressure. The quirk here is that SL tire's load carrying capacity maxes out at 35 psi, where an XL's maxes out at 41 psi.

So that addresses the first question.

The 2nd question? the 44 psi vs 51 psi shouldn't play into which you should be choosing.

And rolling resistance? For a given tire size, RR is a function of the deflection (mostly controlled by the inflation pressure), the amount of material in the tire (especially tread rubber), and the properties of that rubber.

There is a technological triangle for tread rubber compounds involving treadwear, traction (especially wet traction), and RR. To get better values for one, one or both of the others has to be sacrificed. That's why OE tires (the ones that come on new cars from the factory) have such a bad reputation. The car makers want tires with low RR (to be able to advertise better fuel economy) and they don't care much about how many miles the tires last before being worn out, and barely care about traction.

Roundness and footprint don't play much of a role in RR, except to say those impact how much rubber is used.
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Old 04-03-2021, 02:25 PM   #4
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Re: chart for load capacity as function of pressure

Wow. I read all, incl the links to Barry's tire. My truck placard is for 26570R17 40 psi. The tires that are on it are a 2nd set of Goodyear Wrangler, worn out at only 50K miles. Max pressure on those are 44 psi. When I tow, I keep the rear tires at 44, front at 40; but, nevertheless, I see visible deflection in the rear tires with all that weight (despite WD set up properly so both the vehicle and the trailer are level, no sag).


The tire deflection (and the fact the tires wear out in 25K) is what started me researching all of this. My "conclusion" from reading what you both posted seems to point me into these, since maybe (?) I can get better mileage by jacking the psi up to 48. Or maybe not! I don't care about price.

Manufacturer Model Weight LoadIndx warranty psi treaddpth Nokian one ht 39 115 70 51 13 treadwear traction temp price 720 a a $157
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Old 04-03-2021, 05:34 PM   #5
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Re: chart for load capacity as function of pressure

Nokian is a VERY good tire.. who knows more about harsh environments than Finland..
i use their Hakkapeliittas for snow and ice travel here at 9400ft..

--and Capri is very correct: OEM that get shipped with vehicles are the minimum in every standard.. if setting side by side to the same exact brand/model/size tire, and the one on the new car is completely different..

if you are mostly hauling that 6000lb (dry weight, or loaded) trailer; with payload in bed, more than unhitched driving, you might consider jumping up to a load range E/ 10-ply tire.. will hold up to 80psi, and probably carry that weight to at least a 40k mile mark range (i usually get about 60k on load range D or E, but i do NOT carry close to that weight in bed & trailer); however, you will suffer somewhat on mpg..
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Old 04-04-2021, 06:17 AM   #6
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Re: chart for load capacity as function of pressure

Timeking,

It is unusual for a vehicle to specify 40 psi in a P type tire. The only reason I have seen this done is for handling reasons.

You might consider going with an LT tire.
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Last edited by CapriRacer; 04-04-2021 at 06:24 AM. Reason: Found out what vehicle it was by looking at an earlier post!
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Old 04-04-2021, 01:35 PM   #7
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Re: chart for load capacity as function of pressure

There is no LT tire in size 265/75R17 unless I go to E rating off-road, but doesn't that raise concern about running 80 psi for truck placarded for 40 psi? But, then, I'm considering purchase of a 51 psi tire. I don't think I will go off-road and am more concerned about highway towing mpg.


To summarize:


a 51 psi tire will get poorer mpg at 40psi than a 44psi tire.

rolling resistance of 51psi on your rig with moving mass in bed and on tongue will be the best overall for mpg

44 psi vs 51 psi shouldn't play into which you should be choosing.

increasing the inflation pressure increases the maximum load carrying capacity up to a maximum value.

Tire size and pressure info are vehicle specific and second guessing a vehicle engineer is not a good idea. Even after 30 years in the business, I rarely question a vehicle manufacturer's pressure recommendation.

This means you SHOULD NOT use the pressure stamped on the sidewall as any sort of reference point, except, of course, what it says - a maximum.
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Old 04-04-2021, 02:34 PM   #8
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Re: chart for load capacity as function of pressure

timeking,
CapriRacer is right..

a P assenger tire is usually 35psi; they can go higher, but mostly for a lower profile, higher performance/speed rate/etc..
an LT light truck will be from a load range C/D/E, and this corellates to ply; which then increases psi, and then weight, and heat dissipation, etc...

skip whatever the placard in the door jamb says.. this is what it was manufactured with whenever.. just like computers, tyres become antiquated in 6months.. they still work,
but that data plate is stipulating what your rig came with, whether for 89yr old grandma that takes her truck to church on sundays, or the teenager; ricky racin' and bo&luke dukin' out in the bayou..

if you are concerned with high payload weight, over highway surface (mostly), higher mileage than 25,000 and throwaway treads, and not a huge sacrifice on mpg as well;
get the tires you want/prefer/size in and know that the psi stamped on side of that tire is what that manufacturer has tested to max, and will meet or exceed the OEM ones that were inadequate before they were mounted.

here is a rundown of what i think you are looking for:
https://www.treadwright.com/blogs/tr...-vs-load-range
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Old 04-05-2021, 07:13 AM   #9
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Re: chart for load capacity as function of pressure

First, I don't think the size is 265/75R17. I think it is P265/70R17. There are LT's in that "size".

Second, you don't need to inflate a Load Range E to 80 psi. In fact you can inflate to whatever pressure you want, just be aware that the load carrying capacity varies by inflation pressure, so choose the pressure appropriately. In your case, the obvious answer is 40 psi.

The max pressure listed on the sidewall has NOTHING to do with fuel economy. Rolling resistance is mostly controlled by the materials the tire is made out of - mostly the tread rubber compound and the amount of rubber in the tread.

Ergo, an All Terrain tire will have more RR than an All Season tire, all other things being equal (which most of the time those two types of tires also include a different tread compound, the AT having more cut resistance which will hurt RR.)

Also, a tire with a high treadwear rating will have worse RR than one with a lower rating, all other things being equal (and like the example above, those things usually aren't equal!)

Yes, it's complicated.

And just to add clarity, the pressure listed on the vehicle tire placard is applicable for ALL tires in that size, regardless of who makes the tire. You may find that the ride or the handling is different, but that is NOT because of the max pressure. It's due to the way the tire engineer designed the tire - and those things do not affect that load vs inflation pressure relationship.

In this case, I suspect that the reason a 35 psi rated tire is specified at 40 psi is for handling reasons. That increased pressure will obviously hurt the ride and is not needed for load carrying capacity.

I have seen that done before, and it always puzzles me why that happens - except for an 11th hour "fix" before the vehicle production starts.
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Old 04-11-2021, 11:30 AM   #10
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Re: chart for load capacity as function of pressure

So I could buy a set of RoadOne (Pirelli) HT 10 ply tires and safely run them at 40 psi? Or Cooper Discoverer HT3 10 ply? Or the Nokian (51 psi) tires and safely run them at 40 psi?
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Old 04-12-2021, 06:28 AM   #11
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Re: chart for load capacity as function of pressure

Quote:
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So I could buy a set of RoadOne (Pirelli) HT 10 ply tires and safely run them at 40 psi? Or Cooper Discoverer HT3 10 ply? Or the Nokian (51 psi) tires and safely run them at 40 psi?
Lots of caveats, but yes.

One of the caveats is that the tire size has to be what the placard says. If the size is different, then the pressure has to be recalculated. The calculation isn't difficult, but it requires the use of those load tables that started this thread.

Also, the use of the phrase "10 ply" is old and outdated. They now use either "Load Range" or the size will have a "Load Index" after it. The reason it was replaced was because of the confusion created when synthetic fibers came along, which were much stronger, so tires no longer had 10 plies. Even today, people think that a "10 ply" tire has that many plies and think they are getting cheated when they find out that they have only 4!
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Old 04-12-2021, 09:16 AM   #12
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Re: chart for load capacity as function of pressure

https://www.kaltire.com/en/ply-ratin...oad-range.html

http://ittaindia.org/?q=node/85

both Cooper Tire (Akron/Cincinnati/Findlay, OH; now owning Goodyear) and Nokian Tyre (Finland, and subsidary of Bridgestone) are great shoes..

couldn't pay me to put scorpions back on my rig..
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Old 04-15-2021, 10:03 AM   #13
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Re: chart for load capacity as function of pressure

Conflict confusion as follows:


1a. psi should be as close to max as possible to insure best overall mpg,

1b. rolling resistance of 51psi on your rig with moving mass in bed and on tongue will be the best overall for mpg and to assist in removing "roll" or spongy cornering

1c. max pressure listed on the sidewall has NOTHING to do with fuel economy. Rolling resistance is mostly controlled by the materials the tire is made out of



2a. skip whatever the placard in the door jamb says.

2b. the pressure listed on the vehicle tire placard is applicable for ALL tires in that size, regardless of who makes the tire. You may find that the ride or the handling is different, but that is NOT because of the max pressure. It's due to the way the tire engineer designed the tire - and those things do not affect that load vs inflation pressure relationship.



3a. 51 psi tire will get poorer mpg at 40psi than a 44psi tire

3b. 44 psi vs 51 psi shouldn't play into which you should be choosing.
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Old 04-17-2021, 08:23 AM   #14
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Re: chart for load capacity as function of pressure

Quote:
Originally Posted by timeking View Post
Conflict confusion as follows:


1a. psi should be as close to max as possible to insure best overall mpg,

1b. rolling resistance of 51psi on your rig with moving mass in bed and on tongue will be the best overall for mpg and to assist in removing "roll" or spongy cornering

1c. max pressure listed on the sidewall has NOTHING to do with fuel economy. Rolling resistance is mostly controlled by the materials the tire is made out of



2a. skip whatever the placard in the door jamb says.

2b. the pressure listed on the vehicle tire placard is applicable for ALL tires in that size, regardless of who makes the tire. You may find that the ride or the handling is different, but that is NOT because of the max pressure. It's due to the way the tire engineer designed the tire - and those things do not affect that load vs inflation pressure relationship.



3a. 51 psi tire will get poorer mpg at 40psi than a 44psi tire

3b. 44 psi vs 51 psi shouldn't play into which you should be choosing.
First, you have to be careful. Not everyone is an expert, but there are a lot of opinions out there. So you will run into contradictory information.

Second, you have to be careful to look at the details of what statements you find. For example, the RR of a 51 psi LABELED tire (vs a 44 psi labeled tire) is different than the same tire actually using 51 psi (vs one using 44 psi). RR tests are NOT conducted at the max pressure.

So allow me to try to make this clear:

The higher the inflation pressure, the lower the RR. - AND - There is a diminishing return - meaning that the decrease in RR from 35 to 40 psi is smaller than the difference between 30 to 35 psi.

The fact that a tire has a max pressure listed has nothing to do with what the RR will actually be.

Different sizes of the same exact make and model of tire will have different RR values.

You may find comparisons of tire RR's, but those comparisons are based on tests, and there are a few different RR tests and they will give different values. Typically that test specifies a load and inflation pressure. So long as the tests are the same, you can compare them - BUT - you shouldn't compare values where the tests are different.

Tires sold in the US are NOT required to have RR values displayed, listed, specified, etc. EU sold tires do have a label that requires an RR rating, expressed In A, B, C, etc. grades. EU does not require the value to be displayed.

Tire performance (and there are a lot of different parameters of tire performance) can vary a lot between tire make/models - and even within a given brand, the differences can be HUGE!!

In the US every vehicle has to have a vehicle tire placard that specifies the original tire size and specifies a pressure for that size. That pressure is applicable regardless of who makes the tire, because the placard is all about load carrying capacity - which goes back to the tire standardizing organizations.
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Old 09-18-2021, 09:09 PM   #15
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Cool Re: chart for load capacity as function of pressur

Something else I was wondering lately:

To which side will a vehicle pull if the tires on one side are inflated to, IE: 5psi lower than on the other side. 35psi on the left tires, and 30psi in the rights.

My theory: Up to a point, the vehicle will drift/pull to the side with higher tire pressures, because the right-hand tires, at lower pressures, will roll at higher rpm. This might be the case with the right tires deflated to 34, 33psi, until resistance overcomes the higher RPMs, and ultimately, with the right hand tires deflated to 30psi or less, the car starts pulling to the right.

Your thoughts?
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