What air pressure for new tires

[OVERKILL]

You clearly are more knowledgeable about tire construction than I am, but I can't help but point out that all of the above sound to me like there isn't a one-size-fits-all with tires. This is the point I was trying to prove. Every tire is unique, and every driving habit is unique. Therefore a one-size-fits-all door placard is not to be automatically trusted in absolute.

His last point rings true though, the proper inflation pressure (for load carrying capacity, handling, braking and traction) may not result in the perfect wear pattern, because the wear pattern isn't covered under regulation, it's a function of the tire's design, and if the design is subpar in that area, or if the design compromises wear to excel in those other areas, you are going to observe odd patterns in wear, even though the tire is actually performing as it should.

The other points are the identification of caveats that aren't providing leeway on pressure but rather explaining why some tires may wear differently at the proper pressure than others. This isn't a rubber stamping to try and rectify that wear pattern via manipulation of pressure, which will cause changes in performance in other areas, but simply an explanation of why that phenomenon may be observed.

 

[GoldDot40]

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.

Not according to Michelin. The automotive manufacturer door pillar placard is the gospel regardless.
From Michelin's website:

What air pressure do you recommend when using an Optional tire size?

When installing a different size than the original equipment tire, all vehicle manufacturer specifications must be maintained. The replacement tire should be inflated to provide the same load capability of the original tire size at the manufacturer’s recommended pressure.

 

[BillyHW]

Not according to Michelin. The automotive manufacturer door pillar placard is the gospel regardless.
From Michelin's website:

Michelin is saying to do the load table calculation here, although in practice it will not differ substantially from the door placard setting unless you change your tire size radically.

 

[jadatis]

When going from standard load P-tire to E-load LT substantial higher pressure is needed for the same load.

But who is that crazy to make such a change

 

[GoldDot40]

When going from standard load P-tire to E-load LT substantial higher pressure is needed for the same load.

But who is that crazy to make such a change

Not in my experience. I run 285/75 load E tires on my ½ ton Silverado. The factory tire was a 265/75 SL rated. Placard calls for 35psi cold. That's exactly where I run the E rated tires too. I get the best ride, handling and wear from them.

The tire shop that mounted the very 1st set I ran had them on 80psi (idiots). Truck was all over the road...just wandering from lane to lane. Got home and dropped them down to 35 and it was night and day. I'm on my 4th set of E rated tires...and I've ran them at 35psi.

 

[OVERKILL]

Not in my experience. I run 285/75 load E tires on my ½ ton Silverado. The factory tire was a 265/75 SL rated. Placard calls for 35psi cold. That's exactly where I run the E rated tires too. I get the best ride, handling and wear from them.

The tire shop that mounted the very 1st set I ran had them on 80psi (idiots). Truck was all over the road...just wandering from lane to lane. Got home and dropped them down to 35 and it was night and day. I'm on my 4th set of E rated tires...and I've ran them at 35psi.

To have the same load carrying capacity as a P-Metric, an LT will require more pressure. I posted an example from Toyo showing you how to properly calculate this on the previous page. Running an LT at placard means you may be dangerously underinflated.

Let's say for example that your OE tire was a P265/70R17 (2005 Silverado) with a Load Index of 113. That's a capacity of 2,472lbs at 35psi, we need to derate for P-Metric, so this gives us 2,247lbs.

Now, let's say you swapped those out for a Cooper Discoverer AT3 XLT in 285/75R17, which has a load index of 121. That's a capacity of 1,890lbs at 35psi.

Going through the load table, you need to run 45psi to get meet the load carrying capacity of the OE tire:

 

[jadatis]

All true, but the 35 psi is most likely a standard advice and not calcated for the GAWR's and max technical car speed.
Most likely ( but I have to calculate) mayby even 28 psi covers the GAWR's
Then the E load LT needs 38 psi for fully loaded ( estimated) , and you dont drive fully loaded 99% of use.

In Europe , where I live, derating of P tires was never done. But adding 10% reserve to determined loads is for every tire wise, to cover inacuracy's in the measurements.

 

[OVERKILL]

All true, but the 35 psi is most likely a standard advice and not calcated for the GAWR's and max technical car speed.
Most likely ( but I have to calculate) mayby even 28 psi covers the GAWR's
Then the E load LT needs 38 psi for fully loaded ( estimated) , and you dont drive fully loaded 99% of use.

In Europe , where I live, derating of P tires was never done. But adding 10% reserve to determined loads is for every tire wise, to cover inacuracy's in the measurements.

@CapriRacer made the following statement on the previous page:

The pressure listed on the vehicle placard is also derived from the load tables.

Per your point though, he's likely not calculated it at all. He just went by placard because he didn't know any better. Tire pressure has to be THE least understood subject discussed on this forum, it's absolutely wild.

Unfortunately, it's not uncommon for folks to overload 1/2 ton trucks. You combine that with somebody running LT's that are grossly underinflated for the factory load rating and you have a recipe for disaster.

The GAWR for the rear axle on my RAM 1500 is 4,100lbs according to this:
RAM 1500 GVWR, GAWR, curb weight..etc

That would require 40psi in a load range E to meet, and that's with no margin and we know they want margin, so if I suspect if I looked it up, I'd end at the same 45psi I determined for his truck.

 

[4WD]

@CapriRacer made the following statement on the previous page:


Unfortunately, it's not uncommon for folks to overload 1/2 ton trucks. You combine that with somebody running LT's that are grossly underinflated for the factory load rating and you have a recipe for disaster.

Of course many switch the factory P’s to LT’s without understanding the difference …

 

[teh60]

I run 35psi in all my cars.

 

[OVERKILL]

Of course many switch the factory P’s to LT’s without understanding the difference …

Absolutely, see my edit.

 

[billt460]

All this over tire pressure? Imagine if these guys took flying lessons!

 

[jadatis]

Mayby I was generaling to much .
The GAWR rear of 4100lbs +10% reserve is 4510lbs . This gives next calculation for P tire. (4510/( 2x 2472))^1,25 * 36psi= 31.2psi , so not the 28psi in your case.
But there are also cars of wich the OEM P- tires , one can carry almoast the complete GAWR.

Will search back the 285/75R17 you gave with loadindex 121, I think its not an E-load, mayby XL. Or C- load. Will come back to it in next post.
But what is GAWR front.
Normal use no overloading.
In the RV world overloading of single axle rear axle is often seen, but comon use axleweights stay beliw GAWR's.

 

[George Bynum]

The door sticker calls for 30 pounds but a different size tire is now installed

Both are nominally load range 94, so the listed pressures should be good.

 

[OVERKILL]

Mayby I was generaling to much .
The GAWR rear of 4100lbs +10% reserve is 4510lbs . This gives next calculation for P tire. (4510/( 2x 2472))^1,25 * 36psi= 31.2psi , so not the 28psi in your case.
But there are also cars of wich the OEM P- tires , one can carry almoast the complete GAWR.

Sure, but we aren't talking about cars, we are talking about trucks, which is where this whole charade with respect to the fitment of LT tires and improper inflation becomes the issue.

Will search back the 285/75R17 you gave with loadindex 121, I think its not an E-load, mayby XL. Or C- load. Will come back to it in next post.

This is the example tire I used, it is indeed Load Range E:

But what is GAWR front.

For the RAM 1500? It's in that chart I linked you, 3,700lbs.

Normal use no overloading.
In the RV world overloading of single axle rear axle is often seen, but comon use axleweights stay beliw GAWR's.

The point being, if buddy with the LT's has a 4,100lb GAWR for the rear axle, like my RAM, and he's running 35psi, he's got a load carrying capacity of 3,780lbs for the tires. The tires aren't suitable/safe at that pressure to carry the load the truck is rated for.

 

[George Bynum]

Pump them up to 40 PSI and drive them for a while. (That's most likely still well under the max pressure on the sidewall). They'll look better, most likely ride and drive better, and you can happily motor on.

Re: "They'll look better, most likely ride and drive better"

Look better is your opinion, and I won't even consider arguing.

Ride better is IN MY OPINION, not true if better means comfort. Road surface imperfections will be amplified.

Drive better, I'd say absolutely not. Less traction, poorer braking, overly sensitive steering are not better.

@CapriRacer will you please educate both of us?

 

[GoldDot40]

Running an LT at placard means you may be dangerously underinflated.

Going through the load table, you need to run 45psi to get meet the load carrying capacity of the OE tire:

In my experience, any time I've tried to run 40psi or above with the E rated tires, they began to dish in the middle of the tread like any tire would do when they're over-inflated. @35 psi, the treadwear is nice and level across the tread width.

I'm averaging 70K+ miles per set running them @35psi. They don't squat or bulge on the sidewalls whatsoever and have never developed the tell-tale "heat ring" around the sidewall due to too much sidewall flex that you see on underinflated tires. I have NEVER experienced any of that running the psi I run.

So "dangerously underinflated"? Not in my case. There is zero evidence showing my tires are underinflated or unsafe.

 

[CapriRacer]

The "Rule of Thumb" is that the load carrying capacity for an LT tire requires 15 psi more than for a similar sized P type tire. So a 35 psi P tire spec should translate to a 50 psi LT tire. Running 40 psi would be 10 psi too low after such a swap.

 

[OVERKILL]

In my experience, any time I've tried to run 40psi or above with the E rated tires, they began to dish in the middle of the tread like any tire would do when they're over-inflated. @35 psi, the treadwear is nice and level across the tread width.

Curious as to your reason for using a load-range E tire on a 1/2 ton?

I'm averaging 70K+ miles per set running them @35psi. They don't squat or bulge on the sidewalls whatsoever and have never developed the tell-tale "heat ring" around the sidewall due to too much sidewall flex that you see on underinflated tires. I have NEVER experienced any of that running the psi I run.

So "dangerously underinflated"? Not in my case. There is zero evidence showing my tires are underinflated or unsafe.

Dangerously underinflated in terms of the load carrying capacity, as has already been discussed. You can see this just using the numbers I've already posted, by running below 45psi, you have a tire that has a lower load carrying capacity than the truck was shipped with. @CapriRacer has said the same. That doesn't mean the tire is going to sit funny or wear oddly, it simply means it isn't rated to be able to safely handle the same load at that pressure.

 

[Pew]

All this over tire pressure? Imagine if these guys took flying lessons!

Right? Just kick the tire. If it doesn't kick back then you're good!