What do all those letters mean in the tire size?

A fairly consistent problem I try to provide answers about specific tire applications, is the fact that we are usually only given part of the tire information we need to provide an informed answer. I have been asked “What load can my 16” tires carry” or “I have 8 ply tires, how much load can they carry?” or “Can I replace my 235/70R16 tires and carry more load with a 245 tire?”
While I really want to help you get the information and answer you seek, I do not want to guess what tire you have as I may guess wrong and give you incorrect information.

The reality is that the numbers alone are not all the information we need. To help us do a better job of answering your questions, you can help us by giving us the needed details.

Here are some examples of how you can help us all if you provide complete size information when you ask a question.

A P235/75R15 105S is rated at 2,028 Lbs at 35 psi maximum. This would be a “Standard Load” tire, while a P235/75R15 108S XL or Extra Load is rated at 2,183 Lbs at 41 psi max.

We can break down the different parts of the complete size designation as follows. The P stands for Passenger. The 235 is the width of the tire in millimeters and the 75 is the Aspect Ratio of how tall a tire is relative to how wide. In practical terms it is how close the wheel is to the road. The “R” of course stands for Radial and if the tire were Bias as tires were before the introduction of Radials it would have a D for Diagonal. Don’t ask me why they didn’t choose the letter B. I have no idea.

We all know the 15 is the rim diameter. The 105 or 108 is the Load Index. You can consider any passenger tire to be Standard Load unless marked “XL” or “Extra Load”. The “S” in the size above is the speed or handling rating. Passenger tires are not rated for dual application. The combination of Load Index and the Speed rating make up the Service Description.

When you ask a question about a tire don't worry about what all the letters and numbers mean just look at your tire and copy all of the information and leave it up to me and other tire engineers to decipher the mumbo-jumbo. If you tell us your tire size is a P285/35ZR19 87Y we will know this ultra wide, very low aspect ratio large rim tire with a high speed rating indicates you are driving a Corvette and that those tires are not meant to be used on your RV.

Next time we can move on to other type tires that many will find on their Pick-Up, Trailer or RV.

Light Truck tires on trailer

Had a question from a reader
Tireman:
Thanks for your expertise. I am, frankly, baffled about what to do about our tire situation. In this instance, our tire that went flat definitely was not overinflated. It had been checked about two days prior and had 63 pounds. We have been running nitrogen since the tires were put on and have not lost a pound in any of them in 3 months.
When we upgraded to 80 pound, E-rated tires three years ago, I asked about the rims' ability to hold that pressure. Stupidly, I took the tire dealer's word for it.

In 2008, we started blowing Denman tires all over Montana: three of four blew, tires with about 8,000 miles on them.
On our way to Maine in May, we blew yet another tire and sought advice from the guy who owns the campground where we were headed. He is a tire dealer, and advised us to go back to the original equipment 65-pounders, which we did. Now, we have a rim problem.
An 80-pound Maxxis, which we kept as a spare, is now on the trailer. The one that went flat will be on a new rim and used as a spare until we get home. Clearly, we had lost pressure in that tire before we got to the CG; how much is the question. It was not flat or we would have noticed it when we put the chock between the wheels. That tire will undoubtedly go bye-bye when we get home.

I will research rims. It sounds like I should replace all rims with ones that are capable of handling E-rated tires if we want to go that route again. I do not think we have room to upgrade to 16-inch wheels, but I now am going to find out for sure. If it's possible, we will do that.

I am adamant about checking tire pressure, so I am confident we have not been running on over or under inflated tires. We are not overweight, and our axles are fine.

It is obvious we are doing something wrong. I'm beginning to feel like we are too stupid to own this 5er!

Hope your eyes don't glaze over reading all this...thanks for your input.

Answer
You didn't mention if you ran 65psi or 80 psi in your "80 pound" tires. I wonder if you are running metal valves, as standard rubber valves are not rated for more than 65 psi. Also the rims have a max load and max inflation rating. Did you check the rim stamping?

You also were not clear if you were running LT or Light Truck tires or had been running ST or Special Trailer tires. These two different type tires have different load ratings even if the rest of the numbers were the same. A LT235/75R16 Load Range E has a different rating than an ST235/75R16 Load Range E.

An "80 pound" Load Range E tire rated for 80psi will carry no more than a Load Range D tire will if they are both inflated to 65psi. It is the air (nitrogen) that does the work and carries the load not the tire. Just because you checked the tire two days earlier is no guaranty you didn't get a nail as you left the CG that day and drove for two days on a leaking tire. This is one of the best arguments in favor of TPMS.

With all the tire failures I have to ask what your real ( not sticker or guess) axle by axle side to side loads are when you are fully loaded going down the road. With those numbers you can consult the manufacturer’s charts.

While you are getting the TT weighed it won't hurt to get the four corner weights of your tow vehicle too.

One thing few realize, including the tire salesman, is that according to Industry Guidelines LT tires require an inflation increase or even a load reduction if you ever drive over 65mph. ST tires also have a speed restriction that needs to be considered. The only way to know if this is not needed is to consult printed literature for your brand and design tire from the tire manufacturer. I would not assume the tire salesperson knows the correct answer.

Tire Load & Inflation part 2

In part 1 of Tire Loads & Inflation Gene from Idaho questioned how air carried the load not the tire structure.

Think for a moment about a standard balloon. If we increase the inflation pressure, the balloon does not become a square. In fact if we placed the balloon on a table and hold it down with our hand and then increased the pressure we would seen the side bulge out and be more curved. A tire section as seen in this picture (by Pirelli) is much like the cross section of our balloon held between the table (this case the rim) and our hand (the belts). Increasing the pressure will not make the sidewall go straight but in fact when measured will make the width increase due to more curvature. If we check Wikipedia for "Contact Patch" we find "The larger the inflation pressure, the smaller the contact patch".

I will leave you with some additional pictures. You can see how under-inflation will cause the shoulders to wear while over-inflation will cause the center to wear faster.

Here we see the results of a simple test where a chalk line is drawn on the tread. With an over-inflated and then under-inflated condition the tire is driven a few feet. It is easily seen that the center of the tread is supporting the load in the higher pressure (overloaded) condition.

I hope this answer Gene's questions.

Inflate with Nitrogen and there will be MAGIC

Ok so I may have exaggerated a bit, but not by much.   

Previously I started a series on proper inflation of tires. Before I continue lets cover the Nitrogen inflation question.

Nitrogen, Symbol  N  Atomic Number 7   Mass 14.01

Important concept: A claim of “up to” 30% savings can be met with a 0.01% savings. Telling the truth but leaving out the whole truth can be misleading.
When it comes to the suggestion of inflating with Nitrogen there are many claims. Am I suggesting all the claims are false? Not at all, but I believe many are based on assumptions not stated or significant exaggerations of the data that does exist.
Would I see some benefits if I inflate my tires with Nitrogen? The quick and short answer is yes. But, would these improvements be meaningful or cost effective? The answer to this question is probably not.
Warning: What follows is a detailed engineering discussion. Only read if interested. Otherwise you can skip to the end summary.
Let’s look at some claims
Nitrogen is a dry gas and does not contain moisture. A more accurate statement might be that all compressed gas purchased in a metal tank at very high pressure could be considered dry. This would include a tank of CO2, Argon or even a tank of Air.
Oxygen in compressed air contains moisture and is known to cause oxidation. It’s not the O2 that contains moisture. The compressed air we get from an air-line at the local gas station contains more moisture than high pressure gas.
Tires lose air pressure at about 1% to 3% per month when temperature and barometric pressure are held constant. This is true.
Tires inflated with Nitrogen lose pressure more slowly than when inflated with air but they will still lose pressure. True, but they will seldom tell you in the sales pitch that tires inflated with N2 will still loose pressure. The big selling point for Nitrogen is that if you don't check your tires it is a better inflation gas than regular air. But no matter which gas you use, you need to check to ensure you do not have a puncture or leaking valve.
Would you accept the concept that if I use synthetic engine oil I don't have to check the oil level because synthetic lasts longer?
Nitrogen-filled tires run cooler and can improve tire life up to 25%. Engineer Speak Warning. The Thermal Conductivity of pure N2 is 9.8 miliwatts per meter kelvin and O2 is 9.3 making the N2 more thermally conductive by 5%. So the heat generated by the tire due to flexing can be transferred 5% better to the metal wheel for dissipation to outside air. What they fail to mention is that if N2 conducts heat better it will also conduct heat from bearings and brakes to the tire.
Some have claimed fuel economy gains of over 20%. Well here is another claim that is hard to justify if you know how a tire works. It is well established that increase in inflation will lower the fuel usage because the tire deflects less. So, if the claim is that N2 run cooler, how does that not result in a lower pressure in the tire than when the tire runs hotter? Doesn’t it follow that if the pressure is lower, the tire will deflect more which results in greater energy loss due to tire deflection? The only way for a tire inflated with N2 to yield better fuel economy is if you compare it to a tire that has not had its inflation pressure maintained.
Nitrogen does not expand at the rate compressed air does while heating up as you travel down the road.OK now we are really getting technical. First the Gas law needs to be considered. As pointed out this is theoretical but "the ideal gas law is a good approximation for most gases under moderate pressure and temperature." The link also points out "If the temperature changes and the number of gas molecules are kept constant, then either pressure or volume (or both) will change in direct proportion to the temperature". Now the tire is a closed system so the number of gas molecules does not change. There is a slight increase in tire volume as pressure increases and of course this would offset some of the theoretical pressure increase but we can assume that is essentially negligible for the few degrees temperature difference we might be considering. This take us right back to the question of the properties of pure Nitrogen vs. Dry air. This also brings up the question of what percent Nitrogen do you end up with even after a purge with N2. The numbers I have seen indicate you would be going from 78% to 95% N2 not 100%.
Some claim you can reduce tire failures by 50 percent. If this were true don't you think the tire companies would all have been inflating tires with N2 for decades? Where's the data?
Some like to point to aircraft tires. They use N2 but for a different reason. The Autoignition temperature for rubber is 260 - 316°F. Unless there is some mechanical failure you will not see this temperature. I have seen many dozens of tires where components have exceeded 390°F and there has been no fire so it takes a special set of circumstances for this to be an issue.
End Summary
Now having said all this, would it be better to inflate with Nitrogen than air? If there was no cost and you could somehow have N2 available all the time, my answer is yes.
The problem is this is not a perfect world and I don't want to carry around a tank of Nitrogen to inflate my tires or run my air tools as I did with my race car. Even if I was given the tank and regulator for free, hauling it around will take up space and cost me fuel, most likely much more than I might theoretically save with the almost magic properties attributed to this "Wonder Gas".
A lower cost thing to do is to inflate your tires at least 5 psi over that needed to carry the actual load while not exceeding the max for the tire or wheel. If you are concerned about moisture buy some inexpensive air dryer and inflate your tires with dryer air in your line just before your air chuck.. You will gain almost all the anti-corrosion benefits by eliminating the moisture and you can keep these pocket size "filters" almost anywhere if kept in their original packaging. The filter #68215 at Harbor freight is OK for 90 psi and costs less than $8 and another $10 or so will allow you to make up the needed air chuck adapters.

UPDATE:5-17-12 I will do some research and try and find similar dryers for 125 psi application and make a new post.

The questions supporters don’t address are; What is the initial cost? How will you refill your tire pressure as time goes on? Will you be faced with the decision to drive on an under inflated tire to get to some place to inflate with Nitrogen?
What is not stated with all the claims is that if you check your tires for damage and inflation and adjust as you should ie before each trip, you will not see the performance improvements claimed.

Valve placement

Quick post to answer George's question
"Valve extensions for inner dually tires
The awkward placement of valve on outer rear dual tires: why do they face inward and do they HAVE to be that way."

If you look at the valve placement, the "inside" of your outer dual is really the "outside of the wheel if that wheel was on the front or was the inside dual. This means all wheels can be the same. If the valve was on the outside of the outside dual you would need a different spare tire/wheel assembly as you can't have the valve on the inside of your fronts or inside of the inner dual and clear the brakes.

Load Range - Ply Rating - Standard Load and related markings

Had a good question from George. He said "The tires on my Chevy Silverado say "Standard Load" rather than Load Range. What does Standard Load actually mean?"

The simple answer is that your tires are rated equivalent to Load Range B. But I am sure that raises more questions.

This is a good time to clarify some of these terms. To do that we need a bit of a history lesson so we understand why the term "Ply Rating" is out of date.

80 years ago when tires had cotton for the body fabric four Bias plies were needed for use on passenger cars. Trucks needed more air to carry their heavy load which meant more layers or plies were needed to hold the increased air pressure, so truck tires had 6, 8, 10, or more actual layers of cotton body fabric. They needed the even number because they were Bias not Radial construction.

With the invention of synthetic materials such as Nylon, Rayon, Polyester fewer layers or "Plies" of fabric were needed to hold the same air pressure. The term "Ply Rating" came into use when the actual number of plies of these synthetic materials was less than the number of cotton plies.

I remember when I started working in truck tire design in 1969 using terms like "6 for 8" meaning we were using 6 plies of Nylon to deliver the strength of 8 plies.

With the introduction of Radial construction from Europe a new size standard based on actual tire dimensions was used but something was needed to address load so about this time the use of "Ply rating" was replaced with "Load Range" and a letter was used instead of a number as the number was misleading the consumer. We ended up with Load Range, or "LR" B replacing the number 4 and C replacing 6, D replace 8 etc. Since essentially all passenger cars came with Load range B tires it was decided this was the "Standard" load and this did not need to be marked on the tire. There are a few "Extra Load" passenger type tires which you could consider like a LR-C and they are marked as such.

All Light Truck and Heavy Truck tires are marked "Load Range x" with the x being the Load Range for that tire. They may also simply have the Load Range letter right after the size.

To further confuse things there is now a "Service Description" which includes the speed rating. So you may have a passenger tire marked:
P205/75R15 84H The 84 is the "Load Index" and the H is the Speed Symbol.

A Light Truck tire might be LT205/75R15 98/95Q LRC with 98 being the single Load Index and 98 the Dual Load Index and Q the Speed Symbol and LRC indicating the tire is rated a Load Range C tire.

A heavy truck or bus tire might say 255/70R22.5 LR-G 138/134M At this point I trust you can figure out what the letters and numbers mean. If not and want more info check out
this link or simply ask the question.