QuestionQUESTION: I have a 2004 Toyota Sienna minivan. The original tires were Michelin Energy size P215/65R16 96T. I just replaced all four with Michelin X-Radials size 215/65R16 98T. Since the load index is higher, wouldn't that impact the PSI, or should I still go with the door sticker (35 psi)? I know that the new ones are 1653 at 44 PSI, but I'm worried that they're not 1565 at 35 PSI like the originals were. If I inflate to 35 PSI, maybe they won't have the right load capacity?? I didn't like my old tires due to early wear, but I'm concerned that I made a mistake by getting a slightly different load index. If I can't go by the door sticker, I'll have no idea what to inflate the tires to.
Also, the owner's manual says they should be the same "size and construction" as the originals. Would these new tires meet that descrption? Thanks for any help.
ANSWER: Emma,
Technically you are correct - if you change Load Index, you needs to adjust pressure.
BUT
For practical purposes the pressure is the same if the tire size is the same.
But you have the Load / Pressure thing a bit wrong.
The P215/65R16 is indeed 1565 at 35 psi, but the P215/65R16 is not 1653 at 44 psi - it's at 36 psi. - and if you work the math, the 2 tires are nearly at identical loads at 35 psi. (The laws of Physics ought to make that happen - and it does!!)
44 psi is a maximum and I think you'll find the sidewall confirms this.
So for practical purposes, these tires are the "same size and construction".
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QUESTION: Thanks. I'm still a bit confused. Michelin provides a chart that provides the specs for the new X-Radials, and the "maximum load LBS@PSI" column says "1653@44psi". The same chart for the Energy tires says "1565@35psi". How are you figuring the X-Radials are actually "1653@36PSI"? Am I misunderstanding the chart?
ANSWER: Emma,
No, I think you are interpreting the chart fine. I think the chart is in error.
So how do I prove it?
This is going to be hard since I can't show you the reference books from which both Michelin and I obtain these numbers. I guess you'll just have to take my word for it untill you come across a couple of other items that Michelin has done incorrectly (there are several).
Maybe the best approach would be for you to call Michelin and ask them what the load curve looks like for X Radials - you know - a chart that would list various inflation pressures and what the maximum load would be for each pressure. In particular ask them what the load would be at 36 psi.
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QUESTION: You have perfect timing! I just got off the phone with them and then read this. They gave me the max load at 12 different PSIs. It looks like this (I won't give them all): 30 psi = 1421, 31 psi = 1459, 32 psi = 1496, 33 psi = 1533, 34 psi = 1571, 35 psi = 1607, 36 psi = 1644. The loan at 35 psi (the Toyota recommended psi) is said to be 1607 vs 1565 on the original tires. Is that fine since it's higher?
By the way, this is the only X-Radial tire size that does not have a "P" in front of . They said it doesn't matter and that it just means they're European style rather than p-metric size. Huh? Any idea what that's about?
ANSWER: Emma,
If I have this right, you're now convinced the chart has an error in it.
And you asked if I know anything about the difference between tires with "P" in front of the size, and those without.
Yup! I know quite a bit. The US tire standardizing body - The Tire and Rim Association (TRA) uses letters in front of to distinguish between applications. So tires designed for Light Trucks will have "LT" in front and tires designed for Special applications for Trailers will have "ST".
The Europeans have a similar organization called the European Tyre and Rim Technical Organization (ETRTO). They don't use letters, so in some respects it is a bit confusing.
The Japanese have JATMA (Japanese Automobile Tire Manufacturers Association). They also don't use letters.
BTW there are many other tire standardizing organizations, but TRA, ETRTO, and JATMA are the largest and most influential.
Each of these has a slightly different take on the load curves and you can see this if you look at the loads and inflation pressures for what appears to be the "same size".
As I pointed out earlier, the one thing these 3 organizations have in common is that for the same inflation pressure and size, the loads are about the same. They aren't exactly the same, but they are really close. Notice the example you have 1607 vs 1565 - about 3 % different. For practical purposes, "the same".
Why 3 different organizations and 3 different load curves?
Well TRA was one of the first organizations and it was founded in 1930. Each area was pretty isolated in the 1930's, so they developed independently. It has been only in the last 30 years that they have tried to make a single tire standard, but the years of previous work - and the reluctance of people to change - that slows down the progress. The only way we are going to get a single world wide standard is to start with a clean sheet of paper - and that is going to be difficult on the consumer.
Hope that helps.
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QUESTION: This helps a great deal. Thank you. I have one last question for you that is really my bottom line concern. Is it fine that I replaced a p-metric TRA rated "P215/65R16 96T" with a ETRTO rated "215/65R16 98T" tire? Am I bending the rules in any way by not getting a p-metric tire with a "p" in front and a 96T, or is it perfectly fine to do so?
ANSWER: Emma,
Yes, you are "bending" a rule - more like slight "tweak" - but it is a common thing to do and IMHO is an acceptable "tweak" and the "safety" aspect isn't really compromised.
There are times where this would be unacceptable, but this is not one of them.
If you would like I could continue to post why this works - and when it doesn't - but the discussion is pretty long and fairly technical.
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QUESTION: If you're up for it, I would love to hear your why this works, and when it doesn't explanation. I thought for sure you were going to say I wasn't bending any rules. One of the most popular replacement tires for this car, the Hydroedge, is also not a p-metric, and I have never heard mention of that issue in any discussions of it. The whole thing is just strange to me --- that almost all the other X-radial tire sizes would be p-metric, and then you have mine that isn't. If I had it to do again, I would just buy the oem tires. I don't like to even tweak rules related to safety.
ANSWER: Emma,
Before I start, I have to fill you in on something.
As an engineer, I tend to slice the "baloney" thinly and precisely. The net effect is that "technically correct" is an absolute - no exceptions. Let me call this situation A).
But also as an engineer, I recognize that there are things that work, and they kind of fall into 2 categories: B) Things that involve very little risk, and C) things that you can get away with, but in the interest providing the public with a safe environment, shouldn't be done.
If you talk to a tire dealer, they would probably do some things in category C) - even though I personally think they shouldn't.
I could continue to slice this eventually arriving at.....oh, let's call it Z)..... where "It won't run around the block" and will certainly lead to injury or death of the vehicle operator and perhaps by-standers.
To start off with, the 3 tire standardizing bodies evolved different, but similar load curves. This is because the understanding of how tires work was primitive at the time these curves were developed. Many developments have taken place - in particular high speed computers have allowed more detailed and complex analyses to take place and our understanding of the physics involved is much improved.
If we were to start with a clean sheet of paper, the folks involved would agree to a single formula. But as it is, it can not be undone without creating other problems. So we have to live with the differences.
So this makes it not "technically correct" to interchange tires with and without the "P" - Situation A).
But when tires are manufactured, the materials used are standardized in discreet increments, and if additonal strength is needed, the tire is specified with the next higher strength material. This means there will be tire sizes where the material is stronger than the minimum.
Since this is the case with practically all tire sizes, the minor differences caused by the different load curves hardly enters into consideration.
For me this means we have situation B). Other folks would express this differently, but I am afraid I have a very strong need to make this as precise as I can.
This also means that safety is not really an issue.
So when doesn't it work?
The difference between P metric and LT metrics is a good example. Many folks think you can interchange these 2, but they are quite different, and I think this falls into situation C).
Which kind of brings me back to a "Rule of Thumb". It is OK to replace a tire with the same size.
The exceptions are when Extra Load tires are involved or if you are dealing with LT metrics - or in European terms "C" type tires.
If you encouter these situations, which is not all that common, it would be best to deal with a tire professional.
I hope you will forgive me, but I am sure that there will be others who read this thread and there is a point worth making.
You said: "......I don't like to even tweak rules related to safety......"
I hope that means you check your tire pressures on a regular basis. Checking tire pressures is the cheapest and quickest safety check that can be performed. "They" say tire pressures should be checked once a month.
OK, that's a lot of stuff and I hope this has been useful to you.
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QUESTION: What I’m seeing is that all the various p-metric tires in my size have a 96 load rating, while all the Euro sized (non-P) tires of the same size have a 98 load rating. As a non-mechanical person, I take this to mean either one of two things (and I don’t know which is right): 1) that the load curves the Euro ratings people follow cause them to build tires of identical size to the p-metric tires but stronger to hold up to the hire loads their standards require OR 2) that they just give them a higher rating because of how they measure things and they are really the same as p-metric. If the reality is “1” , and my new tires are the same size but stronger, wouldn’t that be a good thing involving no risk and possibly some benefit? If the reality is “2”, then why would it matter at all?
AnswerEmma,
It's more 2 than 1.
Remember I said that I tend to slice the baloney thin. Well this is a point where I do that.
There are some situations where 1 is true - but this tends to be when comparing between manufacturers - and not because of the difference between Euro and the US.
For practical purposes it doesn't matter, but technically there is a very slight difference. And trying to explain all of that is what makes my life interesting.
