What air pressure for new tires

[javacontour]

Load index is key to answering the question.

Not sure if this chart is up-to-date, but this may be a good starting point:

https://www.tsxclub.com/attachments/discount-tire-inflation-1-pdf.47187/

Exactly.

There are load index charts where you can find the original tire's load index and inflation value and look at what load the tire is rated to carry at the intersection of the two values and then find the closest pressure for the non-standard sized tire.


The PDF has an explanation of how to do it.

In your case, the original size seems to come in either 94 or 98 load range. I don't know which is specified for your vehicle. For this example, I'll assume 94. If 98, adjust accordingly.

From the chart, a load range 94 tire at 30psi supports 1270 pounds.

The replacement tire also comes in either 94 or 98 load range, meaning it will support just as much.

Bonus! If your tires match the load range called for by GM, then you can STILL use the pressure on the placard.

But for this example, let's assume that the new tire is a 98 load range tire. In that case, you would go to 27 PSI if I correctly understand as that would support a load of 1296 pounds.

 

[billt460]

You have the size, load range, as well as the air pressure range molded right into the sidewall of every tire manufactured. Including the tires that are shipped on the car when you buy it.

What more do you need to know? That being the case, who cares what the sticker say's?

 

[Propflux01]

You have the size, load range, as well as the air pressure range molded right into the sidewall of every tire manufactured. Including the tires that are shipped on the car when you buy it.

What more do you need to know? That being the case, who cares what the sticker say's?

Usually only the max tire pressure is listed on a tire. Where are you getting a 'range' of pressures?

 

[kschachn]

Usually only the max tire pressure is listed on a tire. Where are you getting a 'range' of pressures?

Likely confused with bicycle tires.

 

[jadatis]

Exactly.

There are load index charts where you can find the original tire's load index and inflation value and look at what load the tire is rated to carry at the intersection of the two values and then find the closest pressure for the non-standard sized tire.


The PDF has an explanation of how to do it.

In your case, the original size seems to come in either 94 or 98 load range. I don't know which is specified for your vehicle. For this example, I'll assume 94. If 98, adjust accordingly.

From the chart, a load range 94 tire at 30psi supports 1270 pounds.

The replacement tire also comes in either 94 or 98 load range, meaning it will support just as much.

Bonus! If your tires match the load range called for by GM, then you can STILL use the pressure on the placard.

But for this example, let's assume that the new tire is a 98 load range tire. In that case, you would go to 27 PSI if I correctly understand as that would support a load of 1296 pounds.

The 98 loadindex of both sises is an XL /reinforced / extraload tire and carries the maxload of 1653lbs AT 42 psi.
The 94 loadindex 1477 AT 36 psi.
Then the 98 needs slightly higer pressure for the same load.

You looked it up in the standard load list 98 LI.
Looked it back 31psi gives 1301lbs in the XL part of that same pdf.
Though probably the 1259lbs AT 30 psi would probably do too.

 

[Blinker]

The door sticker ONLY applies to the exact OEM tires. If you change brand or size of tires, the door sticker is a guideline and no longer applies.

Read that again if confused.

I’d run 33 PSI for a week or two and see how they handle. Check for unusual wear patterns after a few months.

Lol

Replacement tires of the same size, load rating, speed rating as the factory original tire are considered a direct replacement of the factory tire. The factory recommended cold air pressure PSI recommendation is 100000% safe on a direct replacement tire

 

[javacontour]

The 98 loadindex of both sises is an XL /reinforced / extraload tire and carries the maxload of 1653lbs AT 42 psi.
The 94 loadindex 1477 AT 36 psi.
Then the 98 needs slightly higer pressure for the same load.

You looked it up in the standard load list 98 LI.
Looked it back 31psi gives 1301lbs in the XL part of that same pdf.
Though probably the 1259lbs AT 30 psi would probably do too.

Because the tires I looked up where standard load tires not XL.

Had I looked up XL tires, I would have used the XL table.

 

[ARCOgraphite]

You have the size, load range, as well as the air pressure range molded right into the sidewall of every tire manufactured. Including the tires that are shipped on the car when you buy it.

What more do you need to know? That being the case, who cares what the sticker say's?

Just because you have been " running that way for 50 years" doesn't mean you have been doing it properly.
Many tires have 44psig max pressure on the sidewall, and If I had run 39psig all around on my old Honda fit or Yaris I would be off the road in a ditch in the first rain storm on the first corner I took - that's if I didnt pound out my strut top rubber isolators and ball race first. Tires are part of the suspension springing also.

- Ken

 

[billt460]

Usually only the max tire pressure is listed on a tire. Where are you getting a 'range' of pressures?

I've had many sets of tires that say, "Keep Inflated From 30 to 45 PSI When Cold", or typical wording. Not just a maximum pressure.

 

[javacontour]

The 98 loadindex of both sises is an XL /reinforced / extraload tire and carries the maxload of 1653lbs AT 42 psi.
The 94 loadindex 1477 AT 36 psi.
Then the 98 needs slightly higer pressure for the same load.

You looked it up in the standard load list 98 LI.
Looked it back 31psi gives 1301lbs in the XL part of that same pdf.
Though probably the 1259lbs AT 30 psi would probably do too.

And to be fair, looking the tire he actually purchased: https://www.tirerack.com/tires/tire...3!537118804928!!!u!293946777986!&gclsrc=aw.ds

It's a 91SL so the answer is if he's actually using a 91 SL tire, he probably needs to go to 34 PSI.

 

[kschachn]

I've had many sets of tires that say, "Keep Inflated From 30 to 45 PSI When Cold", or typical wording. Not just a maximum pressure.

Many sets? Interesting, I’ve never seen that on any automobile tire I’ve ever owned.

 

[billt460]

Many sets? Interesting, I’ve never seen that on any automobile tire I’ve ever owned.

It's not as prevalent today as it used to be. But just going by max pressure on the sidewall, (if that's all that's listed, and backing off 5 or even 10 PSI), is better than going by the door sticker. Simply because it's common for people change not only size, but load range as well. Especially on trucks.

My truck was delivered with P235 X 15 tires. They weren't even LT rated. I replaced them with 31 X 10.5 X 15's. The door sticker showed an inflation range from 28 to 32 PSI for the P235's. The 31's I had installed had a max inflation pressure of 55 PSI. (The tire dealer pumped 45 PSI into them).

I always go by what the tire states, (or slightly less), not the door jam sticker, because many times it's no longer applicable. And could even be dangerous if minimum door jam pressure was in the tire under maximum load with a larger and higher load range tire.

 

[jadatis]

The maximum cold pressure of normal car tires, is not the pressure the maxload is determined for, called reference-pressure.
Max cold 44 to 51psi and for XL sometimes even 60 psi.
Reference-pressure Standard load 36psi and american system 35 psi, but with the change of calculation for pressure/ loadcapacity lists in 2006, also often 36 psi used, wich gives mostly 1 loadindex step higher . So Capriracer used 93 LI, and found P-tire with that 1LI step lower AT 35 psi, in fact tge same tire, but tiremaker calculated maxload for 35 psi. In practice this has marginal effect on loadcapacity for the pressure.

Xl same story EUR 42psi reference-pressure, USA 41psi.
Give it a year or 10 and 35 and 41 psi you wont see anymore....but
In EUR system exeptions in reference-pressure is seen , 80% HW division ( AR for aspect ratio) also in EUR system 240kPa/ 35psi. And some small sises 220kPa/ 32psi .
Xl exeption AR 80 280kPa/ 41psi.
Difference between reference-pressure ( cold) and max cold pressure is used to highen up the referencepressure for speed (and camberangle ( alignment tires like this /-\ on the axle more then 2 degree ) ) with a system depending on speedcode.
Referencespeed of normal car tires is from Q to V 160kmph/99mph. Above that the referencepressure has to be highened up before putting it in the formula to make the pressure/ loadcapacity lists.
So the loadcapacity's in the pdf given here are good upto 99mph.

 

[Propflux01]

Many sets? Interesting, I’ve never seen that on any automobile tire I’ve ever owned.

Nor have I. Only seen max inflation pressure.

 

[kschachn]

Nor have I. Only seen max inflation pressure.

It's also a bit mystifying why there would be a range rather than a value.

 

[jadatis]

On bicycle tires indeed a range in pressure. I thinks its because they can come loose from rimm when to low pressure.
But cartires dont expand that much under pressure, and mostly have 2 humps ( 2x PANG when mounting) to prevent tire comming loose from rim.
In the official european calculation , I once got hold of, and started my tyrepressure story, its given to not go lower then 150kPa= 22psi, and was always assumed to be to prevent comming from rimm.
But with the 2 humps you can go lower, for offroad sometimes 0.6bar/60kPa/9psi is used, and those situations larger chance on tearing the tire from rimm. Real offroad cars must have the 2 humps, I think.

 

[jadatis]

Because the tires I looked up where standard load tires not XL.

Had I looked up XL tires, I would have used the XL table.

I have not checked if 225/50R17 of topicstarter are 94, 93 or 98, yust assumed you searched it richt.
And as you write both sises are available in 94 and 98 LI , the 98 must be XL.
But on tire-sites not always is given , if its an XL, so you then can conclude its a SL .

Edit: googled it and both 225/50R17 and 215/55R17 are given 94 LI in standard load and 98 LI in Extraload/XL.
Putting P in front it gives the 93 LI , standard load and 97 in XL, wich is because of the 35 psi and 41psi, used instead of 36psi and 42psi, to calculate the maximum load for. Again, in fact the same tire, and sometimes made in the same factory, but only different printed for the other market.

 

[football]

Lol

Replacement tires of the same size, load rating, speed rating as the factory original tire are considered a direct replacement of the factory tire. The factory recommended cold air pressure PSI recommendation is 100000% safe on a direct replacement tire

Not always.

I have seen same size, same load rating, same speed rating tires, installed on a car, with factory door jamb pressures, and the car is unsafe to drive. Unstable. Not safe. Sways all over the road.

Running door jamb pressure on this car, with this set of tires, and you'll be in an accident within weeks. So not safe. Seen it a few times.

If you haven't experienced or seen it yourself, I guess you would just believe the text books. But life teaches lessons, learned with actual experience.

I stand by my comment.

The door jamb ONLY applies to the tires that were on the car at retail delivery. If you change the tires after that, the door jamb is a guide.

 

[CapriRacer]

Perhaps it is time for a bit of a lesson in Tire Engineering:

A P225/50R17 has a load carrying capacity of 1433# @ 35 psi. LI = 93 This would be the US tire standard as published by The Tire and Rim Association.

For the European tire standard, a 225/50R17 has a Load Index of 94 - and you'd think that's different, but LI=94 means the load carrying capacity is 670kg @ 250 kPa or 1477# @ 36.3 psi. If you do the math, those are very close to the same.

When tires are tested, they use a "Step Load Test" where the tire is mounted to the testing pressure, and loaded onto a pulley wheel at 50 mph at 85% of the rated load. If they tire survives 2 hours (100 miles), the load is increased 5% to 90% and the tire is run for another hour. If the tire survives 90% another 5% is added and another hour. And so on until the tire fails.

To pass the US federal regulations, the tire has to exceed 100%, but in order to be acceptable to the US consumer (and pretty much everywhere else in the world), the tire has to be substantially better than that. Those values are trade secrets, so I can't tell you what they are. HOWEVER, I can tell you that the range of values for a large population is 3 steps = 10%. It is that imprecise. So a few pounds (kgs, newtons) or a few psi (kPa, bar) doesn't really change how the tire performs. For practical purposes, a Passenger Car tire with a given set of dimensions has the same load carrying capacity.

And what is printed on the sidewall is from the standard, not from some test result.

BTW, I have NEVER seen a passenger car tire with a range of values ANYWHERE on the sidewall. They are always single values.

 

[BillyHW]

Been running that way for over 50 years, on both bias ply and radial tires. In weather from -20F to over 120F, and never had a blowout or a problem. And have gotten good tire and fuel mileage from doing so.

Far more tires suffer from under inflation than over inflation. And it's one of the main causes for premature tire wear and blowouts.

Just curious, but have you experienced uneven wear doing this?