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Old 10-14-2013, 06:24 PM   #1
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Tire Ratings vs Tire Pressures - My Education

I searched the treads and didn't see anything on LOAD/INFLATION INFORMATION from tire manufacturers so I thought I would post what I have found out, so far:

I downloaded LOAD/INFLATION charts from three manufacturers - Goodyear, Michelin, and Toyo. THE SPECIFICATIONS LISTED FOR ALL THREE ARE IDENTICAL! At the bottom of the Goodyear chart it says, "Industry standards for load & inflation are being revised."

Question - Who handles "industry standards?"

Assumption - Since all three charts indicate the same load capacity at the same tire pressures for LT225/75R16 Light Truck Tires, I assume the ratings fall under "industry standards" and would be the same for all tires sold in the US. Any ideas on this?

Next, I have discovered that E rated LT tires size 225/75R16 are all rated for 2680 pounds at 80 psi. If you decrease the tire pressures to 65psi, each chart shows the tires are now only rated as D !! Here is the Inflation Pressure (PSI) versus load data, as shown:
35psi - 1500 lbs
40psi - 1650 lbs
45psi - 1790 lbs
50psi - 1940(C) lbs
55psi - 2060 lbs
60psi - 2190 lbs
65psi - 2335(D) lbs
70psi - 2440 lbs
75psi - 2560 lbs
80psi - 2680(E) lbs

So now lets look at the "industry standard" chart for the standard TRAILER TIRES Airstream gave me on my 2009 28' International:
Goodyear Marathon ST225/75R15 (trailer service)
Max Speed Rating 65mph
15psi - 1060 lbs (seriously, really?)
20psi - 1260 lbs
25psi - 1430 lbs
30psi - 1600 lbs
35psi - 1760 lbs
40psi - 1880 lbs
45psi - 2020 lbs
50psi - 2150(C) lbs
55psi - 2270 lbs
60psi - 2380 lbs (Airstream's recommended pressure)
65psi - 2540(D) lbs

As you can see, Airstreams recommended pressure does not run the tire up to it's full load rating of D.

So, without getting into factors of safety, kinetic rebound (k factor), rotational moments of inertia :-) or whether the tire is rated for human conveyance, it looks like Airstream wants me to have a capacity of 2380 pounds per tire times four tires. The Gross Vehicle Weight of my trailer is 7300 pounds.

Looking back at the light truck specs it appears my new LT tires would need to be inflated to between 65psi and 70psi for this same load rating.

My only conclusion, so far, is if you have made the switch to 16" light truck tires you should be running higher than 60psi to get the same load rating as the GYM's.

My new 16's just came in. (I bought a set of E-rated Kumho LT's) I'll keep you posted on whether I start shaking doors off their hinges!
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Old 10-14-2013, 07:15 PM   #2
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Good luck with the china kumhoe's.
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Old 10-15-2013, 01:20 AM   #3
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Good luck with the china kumhoe's.
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Old 10-15-2013, 01:32 AM   #4
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Originally Posted by farrel509 View Post
My only conclusion, so far, is if you have made the switch to 16" light truck tires you should be running higher than 60psi to get the same load rating as the GYM's.
Yes, that is true, I am running my new 16" tires at 75psi. I have a single axle FC 20 with about 4400 # on the axel, 700 on the tongue.

It rides very well. By accident I left an open bottle of water on the bathroom countertop the other day, towed 50 miles. The bottle was still sitting there when I stopped for a break. It had not moved or spilled.
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Old 10-15-2013, 05:06 AM   #5
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Yes, that is true, I am running my new 16" tires at 75psi. I have a single axle FC 20 with about 4400 # on the axel, 700 on the tongue.

It rides very well. By accident I left an open bottle of water on the bathroom countertop the other day, towed 50 miles. The bottle was still sitting there when I stopped for a break. It had not moved or spilled.
You must have good driving skills, the glass would slide off the counter at the first corner for me.
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Old 10-15-2013, 07:31 AM   #6
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I have settled in at 75psi for my 16" Michelins after experimenting with 70, 75, and 80 psi. On any given trip I have 7500 - 8000#s on the tires total. (and 850 - 950 on the tongue)
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Old 10-15-2013, 08:27 AM   #7
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Quote:
Originally Posted by farrel509 View Post
I searched the threads and didn't see anything on LOAD/INFLATION INFORMATION from tire manufacturers so I thought I would post what I have found out, so far:

I downloaded LOAD/INFLATION charts from three manufacturers - Goodyear, Michelin, and Toyo. THE SPECIFICATIONS LISTED FOR ALL THREE ARE IDENTICAL! At the bottom of the Goodyear chart it says, "Industry standards for load & inflation are being revised."

Question - Who handles "industry standards?"

Assumption - Since all three charts indicate the same load capacity at the same tire pressures for LT225/75R16 Light Truck Tires, I assume the ratings fall under "industry standards" and would be the same for all tires sold in the US. Any ideas on this?

Next, I have discovered that E rated LT tires size 225/75R16 are all rated for 2680 pounds at 80 psi. If you decrease the tire pressures to 65psi, each chart shows the tires are now only rated as D !! Here is the Inflation Pressure (PSI) versus load data, as shown:
35psi - 1500 lbs
40psi - 1650 lbs
45psi - 1790 lbs
50psi - 1940(C) lbs
55psi - 2060 lbs
60psi - 2190 lbs
65psi - 2335(D) lbs
70psi - 2440 lbs
75psi - 2560 lbs
80psi - 2680(E) lbs

So now lets look at the "industry standard" chart for the standard TRAILER TIRES Airstream gave me on my 2009 28' International:
Goodyear Marathon ST225/75R15 (trailer service)
Max Speed Rating 65mph
15psi - 1060 lbs (seriously, really?)
20psi - 1260 lbs
25psi - 1430 lbs
30psi - 1600 lbs
35psi - 1760 lbs
40psi - 1880 lbs
45psi - 2020 lbs
50psi - 2150(C) lbs
55psi - 2270 lbs
60psi - 2380 lbs (Airstream's recommended pressure)
65psi - 2540(D) lbs

As you can see, Airstreams recommended pressure does not run the tire up to it's full load rating of D.

So, without getting into factors of safety, kinetic rebound (k factor), rotational moments of inertia :-) or whether the tire is rated for human conveyance, it looks like Airstream wants me to have a capacity of 2380 pounds per tire times four tires. The Gross Vehicle Weight of my trailer is 7300 pounds.

Looking back at the light truck specs it appears my new LT tires would need to be inflated to between 65psi and 70psi for this same load rating.

My only conclusion, so far, is if you have made the switch to 16" light truck tires you should be running higher than 60psi to get the same load rating as the GYM's.

My new 16's just came in. (I bought a set of E-rated Kumho LT's) I'll keep you posted on whether I start shaking doors off their hinges!
First, you will want to read my write-up on Tire Standardizing Organizations:

Barry's Tire Tech

Then follow that up with the page on Load Tables:

Barry's Tire Tech

********************

Now that you have read those, allow me to point out an error you made.

The term "Load Range" doesn't mean a fixed load carrying capacity. It means a relative strength level - and the actually load carrying capacity which will vary by tire size.

Then I will point out that ST tires are speed restricted to 65 mph, where LT tires are restricted to some higher speed indicated by the speed rating. (OK, it's not quite as simple as that, but allow me to continue)

Part of what goes into determining the load carrying capacity is speed and the load tables are set up for the speed of the intented service. If you dig around the tire standards, you will repeatedly find references to changes in load carrying capacity for differences in speeds. For example, if you want to raise the speed restriction of an ST tire from 65 mph to 85 mph, you need to increase the inflation pressure 10 psi and reduce the load by 10% (which has the same effect as increasing the inflation pressure another 10 psi.)

So LT tires and ST tires are load rated at different speeds. Put another way, if you were to operate an LT tire in exactly the same way as an ST, the load carrying capacities are the same. - HOWEVER - this is a bit of complication that is beyond most peoples ability to understand - They tend to fall back on the published load tables.

So you may encounter some difficulties with a tire dealer refusing to mount an LT tire in place of an ST.

So, no, if you go from an ST225/75R15 Load Range D operated at 65 psi to an LT225/75R16 Load Range E operated at 65 psi, you gain about a 6% increase in load carrying capacity. If you were to operate the LT225/75R16 at 80 psi, you gain a total of about 22%.
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Old 10-15-2013, 10:10 AM   #8
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Gosh this stuff is going over my head. I am sure all this will make sense to hubby, but meanwhile - I am lost...
The current tires on our Argosy 28 are Good Year 7.00-15LT. "Trailer High Mileage Tubeless White Walls"...
Opinions?
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Old 10-15-2013, 11:04 AM   #9
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Quote:
Originally Posted by farrel509 View Post
I searched the treads and didn't see anything on LOAD/INFLATION INFORMATION from tire manufacturers so I thought I would post what I have found out, so far:

I downloaded LOAD/INFLATION charts from three manufacturers - Goodyear, Michelin, and Toyo. THE SPECIFICATIONS LISTED FOR ALL THREE ARE IDENTICAL! At the bottom of the Goodyear chart it says, "Industry standards for load & inflation are being revised."

Question - Who handles "industry standards?"

Assumption - Since all three charts indicate the same load capacity at the same tire pressures for LT225/75R16 Light Truck Tires, I assume the ratings fall under "industry standards" and would be the same for all tires sold in the US. Any ideas on this?

Next, I have discovered that E rated LT tires size 225/75R16 are all rated for 2680 pounds at 80 psi. If you decrease the tire pressures to 65psi, each chart shows the tires are now only rated as D !! Here is the Inflation Pressure (PSI) versus load data, as shown:

snip

So now lets look at the "industry standard" chart for the standard TRAILER TIRES Airstream gave me on my 2009 28' International:
Goodyear Marathon ST225/75R15 (trailer service)
Max Speed Rating 65mph

snip

As you can see, Airstreams recommended pressure does not run the tire up to it's full load rating of D.

So, without getting into factors of safety, kinetic rebound (k factor), rotational moments of inertia :-) or whether the tire is rated for human conveyance, it looks like Airstream wants me to have a capacity of 2380 pounds per tire times four tires. The Gross Vehicle Weight of my trailer is 7300 pounds.

Looking back at the light truck specs it appears my new LT tires would need to be inflated to between 65psi and 70psi for this same load rating.

My only conclusion, so far, is if you have made the switch to 16" light truck tires you should be running higher than 60psi to get the same load rating as the GYM's.

My new 16's just came in. (I bought a set of E-rated Kumho LT's) I'll keep you posted on whether I start shaking doors off their hinges!
CapriRacer covered a lot of your questions but I have a few other comments.
If you want to know the Load/Inflation numbers for your tires it is best to go to the tables on the web site of the manufacturer. While MOST tires will have identical numbers there are a few exceptions and those exceptions would ONLY apply to tires covered by the MFG of the specific tires you are interested in. You should not use information from Goodyear for any other brand tire. You should not use info from Micheline for any other brand tire. Etc, Etc.

So now you understand that while there are "Industry Standards" you should use the tables from the mfg of your tires. Don't confuse the issue by saying you are referring to "Industry Standards" when establishing the Load/Inflation for Goodyear Marathons. For GY you should use this table. (even if the numbers are the same from another mfg for you do not know when one mfg might change their ratings)

Your "conclusion is if you have made the switch to 16" light truck tires you should be running higher than 60psi to get the same load rating as the GYM's." is correct. You must run at least 70 psi to maintain the 2380# load capacity.

I suggest the best investment you can make is metal valves and a TPM system for the TT & TV to alert you when you have a loss of air. Plus have a gauge you know is accurate.
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Old 10-15-2013, 11:20 AM   #10
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Thank you.
I do not see the 7.00 but I do see the 7.5 So- for E tires, that would give 80 psi. So - is this the correct tire for our Argosy 28?
"
7.50R16LT S 1620 1770 1930
2040(C) 2190 2310 2470(D) 2560 2670 2755(E) "
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Old 10-15-2013, 02:17 PM   #11
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Originally Posted by CapriRacer View Post
Part of what goes into determining the load carrying capacity is speed and the load tables are set up for the speed of the intented service. If you dig around the tire standards, you will repeatedly find references to changes in load carrying capacity for differences in speeds. For example, if you want to raise the speed restriction of an ST tire from 65 mph to 85 mph, you need to increase the inflation pressure 10 psi and reduce the load by 10% (which has the same effect as increasing the inflation pressure another 10 psi.)
Barry, Maybe we need a little more information on this fooling around with tire pressures and vehicle load adjustments to go faster with the ST tires.

Aren't those procedures a responsibility of the tire manufacturer or vehicle manufacturer or both?

The regulation addresses increasing tire pressures for LT tires but clearly indicates such increases must be approved by the tire manufacturer and placed in the vehicle's owner's manual by the vehicle manufacturer.

Goodyear says that procedure for the ST tires is available but I have yet to find any mention of it in any RV trailer owner's manual or on the tire's sidewalls.


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Old 10-15-2013, 03:07 PM   #12
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According to the way industry standards describe the procedures for replacement tires, either the same size or plus sizing them, may require air pressure adjustments to acquire replacement tire load capacities equal to or greater than the Original Equipment (OE) tires. Those type of industry standards will normally come from the Tire & Rim Association and/or the Rubber Manufacturers Association or both.

This is important and often misunderstood. The DOT regulations task vehicle manufacturers to set the cold recommended tire pressures found on your trailer's certification label/tire placard. Those recommended tire pressures provide the necessary load capacity to support your trailer's maximum GAWR. They can best be described as "set in stone".

A brief description of how replacement tires are supposed to have equal load capacities when compared to the OE tires gets muddy when LT/P tires are used to replace ST tires. All three have different air pressure requirements to satisfy GAWR load capacity.

This is where a savvy installer will shine. The fitment of replacement tires may require new rims. Clearances must be closely checked to insure minimum standards are met. EXAMPLE: OE tire size ST225/75R15D with a cold recommended tire pressure of 65 psi produces 2540# of load capacity per tire. When replacing with the LT225/75R16E, a new recommended tire pressure would be 75 psi to produce 2560# of load capacity. The installer would tell the trailer owner the new cold tire pressure for his records and then follow-up by placing an auxiliary tire placard adjacent to the original placard for all subsequent owners.

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Old 10-16-2013, 05:41 AM   #13
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Barry, Maybe we need a little more information on this fooling around with tire pressures and vehicle load adjustments to go faster with the ST tires.

Aren't those procedures a responsibility of the tire manufacturer or vehicle manufacturer or both?....
The tire manufacturer is complying with the industry standard (or should be) and the vehicle manufacturer uses the industry standard to determine the proper inflation pressure. So I tend to see this as 100% the vehicle manufacturer's responsibility. In theory, the vehicle manufacturer should gain acceptance from the tire manufacturer for the fitment, but in practice that isn't the way it works.

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Originally Posted by BlackAcese View Post
.......
Quote:
Originally Posted by BlackAcese View Post
The regulation addresses increasing tire pressures for LT tires but clearly indicates such increases must be approved by the tire manufacturer and placed in the vehicle's owner's manual by the vehicle manufacturer.

Goodyear says that procedure for the ST tires is available but I have yet to find any mention of it in any RV trailer owner's manual or on the tire's sidewalls.


BA
This whole business of the affect speed has on load carrying capacity is muddy. The theory behind the way tires work, backed by both experiments to establish the validity of the theory and the actual experience people have with tires, indicates that this is a "truth". In other words, it's part of the science. The way people react to this science varies.

Personally, I think this is a bit too complex for most people - especially considering there are legal ramifications. Following the industry standard exactly as it is written is always a safe position.

If you've followed the saga about LT's vs ST's (and, BlackAces, I know you have), then you'll remember the discussions about responses from tire manufacturers to questions being posed to them - that is, it depends on how you ask the question. It is very important to realize that printed responses show up in court - and will affect the outcome.

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Originally Posted by 28Argosy View Post
Thank you.
Quote:
Originally Posted by 28Argosy View Post
I do not see the 7.00 but I do see the 7.5 So- for E tires, that would give 80 psi. So - is this the correct tire for our Argosy 28?
"
7.50R16LT S 1620 1770 1930 2040(C) 2190 2310 2470(D) 2560 2670 2755(E) "
First, a 7.00-15 is built to an obsolete tire sizing system and those tires have basically disappeared.

No, a 7.50-16 is NOT an equivalent. Note the last number. that's the diameter of the wheel - 15" vs 16"!! 15" tires fit only on 15" wheels, and 16" tires only fit on 16" wheels.

And here is where is gets complicated.

7.00-15's came in "Ply Ratings" - now "Load Ranges" - and that greatly affects what is equivalent.

Dimensionwise, a 7.00-15 is 8.0" wide on a 5 1/2" rim, with an allowable rim width range of 5" to 6 1/2", a diameter of 29.6" with a load carrying capacity of 1710# at 45 psi (LR C) to 2335# at 75 psi (LR E).

An ST225/75R15 is 8.8" wide on a 6" rim (8.6" on a 5 1/2" rim), and allowable rim width range of 6" to 7", a diameter of 28.3", and a load carrying capacity of 2150# at 50 psi (LR C) to 2830# at 80 psi (LR E)

AN ST205/75R15 is 8.0" wide on a 5 1/2" rim, with an allowable rim width range of 5" to 6 1/2", a diameter of 27.1", and a load carrying capacity of 1820# at 50 psi (LR C) and 2150# at 65 (LR D)

So it sounds like an ST205/75R15 is the closest match, but there is a 2 1/2" difference in diameter.

Problem: We don't know which load range is the correct one. You could look at the old tires and see what it says - or you could consult the vehicle tire placard and the owners manual (but I don't think that will yield any useful information) - or you could weigh the trailer (which I would recommend anyway as things have changed over the years.)

Weigh your trailer in the worst possible condition - fully loaded with everything you can think of including full water tanks. Weigh the trailer tire by tire if possible. If you can't I'd recommend adding a factor of 10% to the axle weights, of if you weighed both axles at the same time 15%.

I'd also recommend using no more than 85% of the rated capacity of the ST tires.
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Old 10-16-2013, 05:53 AM   #14
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Thank you for the specific explanation on my tire situation.
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