Tire pressure delima

[Sienna Driver]

The tire on my 95 Toyota Sienna is Michelin Energy LX4 with max pressure 35 PSI, but the recommended pressure is also 35 PSI. Now why would Toyota put the tires on the car with max pressure=recommended pressure?

I have observed the pressure sometime goes up to 38 psi due to temperature changes. Is this safe?

 

[wantin150]

From what I understand, the normal increase in pressure due to heat (driving) is fine. Capriracer is one of the resident experts and he will probably chime in. From what I have gathered in some of his previous posts is that there is a bit of a margin built in. I used to run a set at 44 psi (max for tire), drove them to NY, pretty much non-stop, and was fine. You should be OK unless you grossly overload your vehicle.

 

[Sienna Driver]

Sorry I wasn't clear, when I said temperature change I actually meant weather changes. For example, after I inflated the tires to their proper pressure (35 in this case), a few days/weeks later the whether is unseasonably warm, the cold tire pressure would go up to 38 sometimes. I'd rather not have to worry about this type of things.

 

[John K]

I don't think the increase is significant. The max pressure usually(?) corresponds to the maximum load rating, so if your tire pressure increased significantly and you were carrying maximum load in the van you might have a problem. Might want to monitor your tread wear and make sure you don't show wear in the center, in which case you should run lower pressures.

 

[rriddle3]

Remember, when talking tire pressure, it's always about cold pressure. Whether you're dealing with recommended or maximum, it's the pressure taken when the tire has been sitting at least 4 hours or driven less than 1 mile. The manufacturers know that a tire's pressure will increase as the heat in it builds up. Thus, there is a safety margin built into the maximum rating.

 

[rpn453]

Don't worry about it. A few psi extra isn't going to hurt anything.

 

[CapriRacer]

Quote:

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............The max pressure usually(?) corresponds to the maximum load rating.........

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Sorry, but that is usually NOT the case.

Here's the way it works.

The load / inflation pressure relationship is defined by a standardizing organization. In the US, it's The Tire and Rim Association (TRA).

For P metric Standard Load tires, the maximum load carrying capacity occurs at 35 psi. However, there are certain circumstances where additional pressure is called for - high speed operation being one of them. So it is specified by TRA that the maximum pressures put on the sidewall can also be either 44 psi or 51 psi. Please note that the maximum load doesn't change in these circumstances - and in some cases the maximum load is reduced.

Pretty much every tire manufacturer specifies a max of 44 psi for their S and T rated tires - and there is some disagreement about H and above, but generally it's 51 psi.

HOWEVER, the Michelin group (brand names Michelin, Uniroyal, Goodrich) lists 35 psi for their S and T speed rated products. They are not alone, but the folks that do this are by far in the minority.

As the OP indicates, this creates some confusion.

So let me state this clearly. The pressure listed on the sidewall of a tire is a maximum usage pressure.

EXCEPT!!!

You can inflate passenger car tires up to 44 psi even if the tire says 35 psi max!

A natural consequence of designing passenger car tires results in tires whose bursting pressure is well over 100 psi - regardless of what the maximum pressure is listed on the sidewall.

As indicated in an earlier posting, all these pressures are COLD. The only time anyone talks about tire pressures other than cold is when we are discussing pressure build up due to operation - and it is obvious when that is occurring.

Also, there are such things as Extra Load (XL) Passenger Car tires, which results in more load carrying capacity for a given size, but the usage pressure has to be increased to a maximum of 41 psi. Please don't confuse tires that list 44 psi on the sidewall with XL tires.

Hope this helps claify the situation.

 

[Tempest]

What about for spare tires? I recently had an occasion to use my (much neglected) full size spare, and when I checked the psi (happened to have a gauge with me), it said 0.

Not a good thing when you need it. Well, I put it on anyway (the flat was REALLY flat, down to the rim) and I checked it and it was ~10psi after mounting. Thankfully there was a gas station across the street with an air pump.

Does this mean that spares should be inflated ~10psi less than the running tires?

 

[CapriRacer]

Quote:

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What about for spare tires? I recently had an occasion to use my (much neglected) full size spare, and when I checked the psi (happened to have a gauge with me), it said 0.

Not a good thing when you need it. Well, I put it on anyway (the flat was REALLY flat, down to the rim) and I checked it and it was ~10psi after mounting. Thankfully there was a gas station across the street with an air pump.

Does this mean that spares should be inflated ~10psi less than the running tires?

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No!!!

What you did was take a sealed container and reduced its volume, thereby generating pressure - basically an experiment in the ideal gas law - PV=nRT.

Try this: Take a tire - any tire will do - and measure the pressure, both loaded and unloaded (You'll need a gauge that can read in 0.1 psi or better - that would be technically called a gauge with high "precision".)

The loaded pressure will be slightly higher than the loaded pressure - and the more load, the greater the difference. This means the volume of the tire is smaller when the tire is loaded.

 

[Tempest]

Quote:

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and the more load, the greater the difference

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Exactly my question. A spare tire has no "load" (reduction in volume from the weight of the car) on it, unlike the running tires. So if both are filled to the same psi, the spare psi would be higher than the other running tires after mounting. Just wondering if there is a standard lower pressure, or if the increase in psi is so marginal that it does not matter? If you are talking .1 psi increments of differential, it seems like the latter.

 

[CapriRacer]

Quote:

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Exactly my question. A spare tire has no "load" (reduction in volume from the weight of the car) on it, unlike the running tires. So if both are filled to the same psi, the spare psi would be higher than the other running tires after mounting. Just wondering if there is a standard lower pressure, or if the increase in psi is so marginal that it does not matter? If you are talking .1 psi increments of differential, it seems like the latter.

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It is indeed marginal - at most 0.5 psi - which is why you need a sensitive pressure gauge to be able to observe the difference.

 

[mstrjon32]

I have a donut spare, indicates that it should be filled to 60 psi. I only check it maybe twice a year, but when I do I inflate it to 70-75 psi, as it tends to naturally lose pressure and I don't want to be in the middle of no where with a flat and a spare with not enough pressure. It's easy enough to let it down to 60 psi if I need to use it.

Every time I've ever borrowed/driven someone else's car long enough/helped a friend change a tire; the spare has been totally flat. It's a very important thing to check and no one ever thinks to do it!

 

[itchrelief]

Quote:

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It is indeed marginal - at most 0.5 psi - which is why you need a sensitive pressure gauge to be able to observe the difference.

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Maybe the very low pressure tire "squishes" down more, and its pressure change is more noticeable as the volume change is also greater.

A properly-inflated unloaded spare at 60psi or whatever would probably have so little give that the pressure change would be immaterial.

 

[itchrelief]

oops. Was thinking of a mini-me spare, thus the 60psi. Guess the same principle works for 35-ish psi full size spares too, though.

 

[VeeDubb]

CapriRacer,
My Passat comes with 97LI XL rated tires. Are there any negative consequences to running lower LI tires? I can't find good 97 LI tires that are the appropriate size. The tires I want are only available in 94 or lower.
Thanks

 

[CapriRacer]

VeeDub

I did a quick look at Tire Rack (based on a couple of assumptions) and I found quite a few tires with a 97 LI. So perhaps we ought to go back to the basics - tire size and what you consider makes a tire "good".

 

[XS650]

Quote:

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What about for spare tires? I recently had an occasion to use my (much neglected) full size spare, and when I checked the psi (happened to have a gauge with me), it said 0.

Not a good thing when you need it. Well, I put it on anyway (the flat was REALLY flat, down to the rim) and I checked it and it was ~10psi after mounting. Thankfully there was a gas station across the street with an air pump.

Does this mean that spares should be inflated ~10psi less than the running tires?

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Always inflate the spare tire to the pressure on the sidewall, that way there is a greater chance there will still be some air in it when you need it.

The pressure will increase a lot less when you set a car down on an inflated tire because the tire doesn't change shape as much.

 

[VeeDubb]

Quote:

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VeeDub

I did a quick look at Tire Rack (based on a couple of assumptions) and I found quite a few tires with a 97 LI. So perhaps we ought to go back to the basics - tire size and what you consider makes a tire "good".

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The tire size is 215-55-16 and I'm looking for all season performance tires. The only all season performance tire with 97 LI, according to Tirerack, is Pirelli P6 four seasons which lagged both in tirerack testing and consumer surveys. If I could run 93 LI tires, that would open up a world of options.

 

[Shannow]

As to the spare thing, there are two factors that come into equilibrium when the "empty" spare is put on the wheel.

First is the ideal gas equation P1V1 = P2V2 (assuming T is constant...P is absolute, i.e. gauge plus 14.5 psi). So the tyre squishing reduces the volume, increasing the pressure.

Then there's contact patch, which is holding up the car (neglecting tyre stiffness). The pressure in the tyre x the area of the tyre in contact with the road HAS to match the wheel load.

So if the tyre rose to 10psi, then the contact area (in square inches) must have been the wheel load, divided by 10. Also, the reduction in tyre internal volume must have been (at sea level) have been by 60% to cause the pressure rise seen. I think the 10psi is optimistic.

Extending from this, if the tyre starts at a higher initial pressure, the change in surface area and internal volume is far less, and thus the change in pressure is less.

 

[CapriRacer]

Quote:

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...............Then there's contact patch, which is holding up the car (neglecting tyre stiffness). The pressure in the tyre x the area of the tyre in contact with the road HAS to match the wheel load........

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You were going good until you got to this point - and don't feel bad - this is a common misconception. The average footprint pressure is not equal to the inflation pressure. They are related in that more inflation pressure results in a smaller footprint, however, it is merely coincidental if the 2 pressures are the same. Please note that this does not involve tire stiffness. It is a geometry problem and the false assumption is that the inflation pressure is holding up the vehicle - the inflation pressure is a stiffening agent for the tire's structure.

But to answer VeeDub's question:

Quote:

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......The tire size is 215-55-16 and I'm looking for all season performance tires. The only all season performance tire with 97 LI, according to Tirerack, is Pirelli P6 four seasons which lagged both in tirerack testing and consumer surveys. If I could run 93 LI tires, that would open up a world of options....

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Tire Guides lists the inflation pressure as 35 psi. But I know that German cars list several pressures - and the circumstances for their use - on the placard. I'm guessing that the placard will show something like 40 psi for fully loaded, and perhaps in combination with high speed. Please check on this, as this is critical to the next step.

If this is true - and it would be extremely helpful if you would post what the vehicle placard says - then so long as you don't load the vehicle, a 93 LI should be OK.

But this is a really risky move as I can not imagine a vehicle not being loaded up from time to time. And this is particularly so if you live in a hot climate - you don't, so that's a factor in your favor.

So post what the vehicle placard says and we'll continue the discussion.