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Old 06-28-2019, 08:21 AM   #16
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Re: For Anyone Who Owns a Digital Tire Gauge

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Originally Posted by RidingOnRailz View Post
...…… My friend's AWD 2005 Subaru calls for 34psi front, and 35 rear.

However, for reasons yet unknown, the formula returned suggested cold psi of 48 front and 49 rear! ……..
Remember when I said that the max pressure is NOT related to the max load? I suspect the answer you are getting for the Subaru is because the tire's max pressure is 51 psi, where the actual rating pressure is 35 psi.
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Old 06-28-2019, 08:31 AM   #17
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Cool Re: For Anyone Who Owns a Digital Tire Gauge

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Originally Posted by CapriRacer View Post
Remember when I said that the max pressure is NOT related to the max load? I suspect the answer you are getting for the Subaru is because the tire's max pressure is 51 psi, where the actual rating pressure is 35 psi.
So if I plug 35psi max cold, or even 44(my Hyundai tires) into the formula, I might get actual sane recommended pressures for that Legacy?

And secondly, it's 2019, not 1989: I haven't seen a Max Cold PSI of 35 stamped on a passenger tire since nearly back to then. Since the '90s, 44 and 51psi max cold appear on every pax tire I've had the pleasure of kneeling before.

Are you suggesting that those current max cold stampings might lack some validity?
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Old 06-28-2019, 10:04 AM   #18
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Cool Re: For Anyone Who Owns a Digital Tire Gauge

Quote:
Originally Posted by CapriRacer View Post
Remember when I said that the max pressure is NOT related to the max load? I suspect the answer you are getting for the Subaru is because the tire's max pressure is 51 psi, where the actual rating pressure is 35 psi.
Wait - HOLLLLLLLLD everything!

I just noticed something odd: The USTA load stamped on the Subaru tires(1201lbs) is LOWER than it is on my SMALLER Hyundai(1356lbs), and even smallllller Corolla, also 1356lbs. I'm surprised - and actually embarrassed - that I didn't pick up on that weeks ago!

This changes everything! Apparently her tires are NOT of the OEM specified on her door B-pillar: "215/45ZR17". I neglected to photograph the size stamped on the tires, so while I do recall the size, I don't remember exactly what speed/load ratings the tire place sold someone who owned the car before my friend! Will it be a risk for her to keep these lower load, NON-oem tires on her car, since it is mostly just her driving across town to work, and sometimes with her kid?

So I found a Continental Extreme Contact in that exact size as indicated on the Subaru door sticker, BUT: That "Z" is NOT the speed rating! The speed rating, acc. to Continental's own page for that tire, is W! With a load index of 91(1,356lbs).

So I don't know WHY there is a Z in the middle of that tire size if it does not refer to the speed rating. As Trump would say: "Misleading - VERRY misleading!"


Anywho, I ran the formula with the new 1,356lb USTA figure, and get something a *little* less truckish for my friend's poor Legacy:

F: 43psi, R: 44.


Also Capri, when I left in the original USTA load - the one actually on her car - and used 35psi max cold in the formula, I got:

F: 33psi, R: 34 (basically the opposite of her door placard: F: 35, R: 33).


When I used the correct USTA load(1,356) annnd 35 max cold(as you mentioned), I got:

F: 29psi R: 30

BOTH axles calculated under the door sticker cold pressures! Although those are the exact cold "normal load" pressures I saw on my boss's 2015 Forester. Cosmic.....


So the second to last set of pressures seem the most sensible, to me anyway, for this 2005 Legacy sedan. LOL! Notice though, how in each scenario, the formula flips the pressure difference between front and rear. Interesting.


Long n short: The tire pressure formula I shared happens to work perfectly with the tires on the cars in my household, adding pressure to the heavier axle and subtracting it from the lighter one.

Now for the greatest mystery of the tire universe:

How, in 1994, Ford came up with the recommendations for my 1996 Contour - remember, Mondeo? World car? Driver extraordinaire?

Front: 31psi, Rear: 34!

Wait, wasn't Mondeo/Contour a FRONT-drive econobox - or econobubble?

Why would Ford specify higher tire pressures on the LIGHTER axle?

Anywho, from 20 to 17 years ago, when I drove the Contour, I tried Ford's pressures for one week - and never looked back. One of my favorite drivers! If it had a moonroof, I might still be driving it today!

Last edited by RidingOnRailz; 06-28-2019 at 11:05 AM.
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Old 06-29-2019, 08:44 AM   #19
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Re: For Anyone Who Owns a Digital Tire Gauge

Whoa!! So much to unpack.

Let's start by having you read this web page: http://barrystiretech.com/loadtables.html

This will ask you if you read the page on Tire Standardizing Organizations. Do that, too.

3 things you should pick up:

1) That the rating pressure for Standard Load P metric tires is 35 psi - and even though the max pressures may be at 44 or 51 psi, this value still holds.

2) That there is a note that it is PERMISSIBLE to use 44 psi and 51 psi - hence some tire manufacturers will list one of those as the max pressure. So don't read too much into that, because it doesn't say when to use either, so the decision is relatively arbitrary.

3) That the load curve is NOT proportion to the inflation pressure, so the formula you are using is wrong.


And a couple of tidbits:

What is the USTA? Did you mean the USTMA - US Tire Manufacturers Association? If so, then be aware they don't publish load and inflation information - but the tire standardizing organizations do, hence why I want you to read my page on the subject.

The 1996 Ford Contour? Only the SE version had that pressure split. All the other versions used the same pressure front to rear. (Source: The Old Timers Tire Guide)

Why would Ford do that? It has been known for vehicle manufacturers to tweak the tire pressure to get certain handling characteristics. My opinion is that they shouldn't do this, but they don't listen to me.

There are more things I want to talk about, but I don't want to overload you with information, so I'm doing this in steps.
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Old 06-30-2019, 12:47 AM   #20
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Cool Re: For Anyone Who Owns a Digital Tire Gauge

CapriRacer wrote: "The 1996 Ford Contour? Only the SE
version had that pressure split. All the other versions used
the same pressure front to rear. (Source: The Old Timers Tire
Guide)"

ERRRR - Wrong!

My GL also had that Front/Rear pressure difference on its B-pillar placard, and I went by it.

So I suppose you'll accuse Ford of putting the wrong tire decal on the wrong trim-level Contour.



Yes, I was using USTA arbitrarily - If you suffered the damage I did during youth you might, also.

So exactly who determines those max loads, and thus max cold pressures(35, 44, 51, etc) that get stamped on tires?

By the way, your load tables link is more a text book explaining load theory. I was expecting an actual table where I could look up my specific tire size - or that on my friend's Subaru - and glean some actual load data from it.

It just shows an example of a load table, which helps only those whose tires happen to be listed in that excerpt.
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Old 06-30-2019, 08:50 AM   #21
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Re: For Anyone Who Owns a Digital Tire Gauge

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Originally Posted by RidingOnRailz View Post
…… So I suppose you'll accuse Ford of putting the wrong tire decal on the wrong trim-level Contour. ….
No, I'll be accusing Tire Guides of making a mistake in their book. This wouldn't be the first time I found one.

I wonder what the placards actually say for the other versions. Put another way, it is unusual to specify more inflation pressure for the rear tires on a FWD, and if all the Contours had that pressure split, why did Tire Guides only indicate it for the one combination? I wonder if it was that way for the 1995 models and the 15" - which only came on the 1995 SE model - had that pressure split and Tire Guides just didn't pick up on the change made in later model years.

Quote:
Originally Posted by RidingOnRailz View Post
……Yes, I was using USTA arbitrarily - If you suffered the damage I did during youth you might, also.

So exactly who determines those max loads, and thus max cold pressures(35, 44, 51, etc) that get stamped on tires? ……..
The tire standardizing organizations publish load tables - max load vs inflation pressures - in the form of a book published every year. While a tire manufacturer is not under any obligation to follow those, not only would it be foolish not to do so, there have only been a few cases where the tire manufacturer did something other than what was published - and eventually, those exceptions disappeared.

The US government accepts those standards and requires the tire manufacturers to imprint the max load and the max inflation pressure on the sidewall of each tire designated for street usage.

And here's where the confusion comes in: Officially, all Standard Load passenger type tires will have their load max out at 35 psi (or 2.5 bar (36.3 psi) if the standard is written in metric units), the tire standards allow 44 or 51 psi (3.0 bar and 3.5 bar) to be used as well - and the tire manufacturer deicides what to place on the sidewall. The sort of exception to this is that for the speed rating test, while S and T rated tires are tested at 35 psi, H rated tires are tested at 44 psi and V and higher rated tires are tested at 51 psi. Needless to say, tires with those speed rating HAVE to have max pressure of at least that value.

Note: The speed rating test is the only standard test where the inflation pressures are as indicated in the above paragraph. Standard load tests use 35 psi or 2.5 bar, whichever is appropriate.

Quote:
Originally Posted by RidingOnRailz View Post
……By the way, your load tables link is more a text book explaining load theory. I was expecting an actual table where I could look up my specific tire size - or that on my friend's Subaru - and glean some actual load data from it.

It just shows an example of a load table, which helps only those whose tires happen to be listed in that excerpt.
Unfortunately, those tables are copyrighted and can not be reproduced without permission - which I don't have. You will occasionally find portions of those load tables published by tire manufacturers on the web. But since there is a yearbook published every year with new sizes (and old sizes removed), you will not always find a particular size - although the information on a given size doesn't change from year to year.

Besides, the current TRA yearbook has 60 pages for the passenger tire section alone. For me to publish those would be a lot of work and I risk a copyright infringement lawsuit. Not to mention there are 3 major tire yearbooks in common usage with about the same number of pages - also covered by copyright law.

However, I did publish a few pages from the TRA yearbook for educational purposes (allowed under copyright law), and pages of old yearbooks for obsolete tire sizes - like size G78-15 or 7.35-15. Those old pages don't represent much of a legal risk as TRA would have to show monetary damage to their business and information about old tire sizes just doesn't sell.

Now allow me to return to an earlier conversation we were having:
Quote:
Originally Posted by RidingOnRailz View Post
…… So I found a Continental Extreme Contact in that exact size as indicated on the Subaru door sticker, BUT: That "Z" is NOT the speed rating! The speed rating, acc. to Continental's own page for that tire, is W! With a load index of 91(1,356lbs).

So I don't know WHY there is a Z in the middle of that tire size if it does not refer to the speed rating. …..
At one point in time, there were only 3 speed ratings and Z was the highest - above V - and the speed rating was to be imbedded in the size designation. But when W and Y speed ratings were added, the Z speed rating couldn't be changed, so technically Z speed ratings include W and Y, but are further restrictions - plus it is still permissible for tire manufacturers to place the speed rating in the size if they want to, but with the advent of service descriptions, there is no longer the need - EXCEPT, it seems that some tire manufacturers imbed the Z in the size for W and Y speed rated tires. I don't understand why.
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Old 06-30-2019, 11:21 AM   #22
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Cool Re: For Anyone Who Owns a Digital Tire Gauge

See my 'Railz' responses, within:

Quote:
Originally Posted by CapriRacer View Post
No, I'll be accusing Tire Guides of making a mistake in their book. This wouldn't be the first time I found one.

I wonder what the placards actually say for the other versions. Put another way, it is unusual to specify more inflation pressure for the rear tires on a FWD, and if all the Contours had that pressure split, why did Tire Guides only indicate it for the one combination? I wonder if it was that way for the 1995 models and the 15" - which only came on the 1995 SE model - had that pressure split and Tire Guides just didn't pick up on the change made in later model years.

Railz: My 1996 GL had 205/60R15 tires on it. At this point I don't recall the speed rating, but I replaced them with H-rated, same size.



The tire standardizing organizations publish load tables - max load vs inflation pressures - in the form of a book published every year. While a tire manufacturer is not under any obligation to follow those, not only would it be foolish not to do so, there have only been a few cases where the tire manufacturer did something other than what was published - and eventually, those exceptions disappeared.

The US government accepts those standards and requires the tire manufacturers to imprint the max load and the max inflation pressure on the sidewall of each tire designated for street usage.

And here's where the confusion comes in: Officially, all Standard Load passenger type tires will have their load max out at 35 psi (or 2.5 bar (36.3 psi) if the standard is written in metric units), the tire standards allow 44 or 51 psi (3.0 bar and 3.5 bar) to be used as well - and the tire manufacturer deicides what to place on the sidewall. The sort of exception to this is that for the speed rating test, while S and T rated tires are tested at 35 psi, H rated tires are tested at 44 psi and V and higher rated tires are tested at 51 psi. Needless to say, tires with those speed rating HAVE to have max pressure of at least that value.

Note: The speed rating test is the only standard test where the inflation pressures are as indicated in the above paragraph. Standard load tests use 35 psi or 2.5 bar, whichever is appropriate.

Railz: So the max cold stamped on a tire(35, 44, 51psi) is based on that speed rating?


Unfortunately, those tables are copyrighted and can not be reproduced without permission - which I don't have. You will occasionally find portions of those load tables published by tire manufacturers on the web. But since there is a yearbook published every year with new sizes (and old sizes removed), you will not always find a particular size - although the information on a given size doesn't change from year to year.

Railz: Communists!

Besides, the current TRA yearbook has 60 pages for the passenger tire section alone. For me to publish those would be a lot of work and I risk a copyright infringement lawsuit. Not to mention there are 3 major tire yearbooks in common usage with about the same number of pages - also covered by copyright law.

However, I did publish a few pages from the TRA yearbook for educational purposes (allowed under copyright law), and pages of old yearbooks for obsolete tire sizes - like size G78-15 or 7.35-15. Those old pages don't represent much of a legal risk as TRA would have to show monetary damage to their business and information about old tire sizes just doesn't sell.

Now allow me to return to an earlier conversation we were having:


At one point in time, there were only 3 speed ratings and Z was the highest - above V - and the speed rating was to be imbedded in the size designation. But when W and Y speed ratings were added, the Z speed rating couldn't be changed, so technically Z speed ratings include W and Y, but are further restrictions - plus it is still permissible for tire manufacturers to place the speed rating in the size if they want to, but with the advent of service descriptions, there is no longer the need - EXCEPT, it seems that some tire manufacturers imbed the Z in the size for W and Y speed rated tires. I don't understand why.
Railz: I will have to look at the size section on her sidewall again for that load index and speed letter, IE: '89T', 91V' or which ever hers might be. BTW on my Contour, the replacement tires did have the H embedded inside the size, and they were in fact H-rated tires.


So Capri: Would you consider it just happy coincidence that the formula I shared here worked for mine and my wife's cars and tires, yielding as it did a perfect split just above and below the equal recommended tire pressure on our door placards? Because we both really like how our cars drive after setting them to the values I derived.
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Old 07-01-2019, 08:23 AM   #23
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Re: For Anyone Who Owns a Digital Tire Gauge

Quote:
Originally Posted by RidingOnRailz View Post
See my 'Railz' responses, within:

…… Railz: My 1996 GL had 205/60R15 tires on it. At this point I don't recall the speed rating, but I replaced them with H-rated, same size. …..
That would seem to confirm that it is the 15" option that has the pressure split


Quote:
Originally Posted by RidingOnRailz View Post
……… Railz: So the max cold stamped on a tire(35, 44, 51psi) is based on that speed rating? ……..
Oh, if it were just that simple. The tire manufacturer is the one who decides what max pressure is to be imprinted and he is constrained to use those 3 pressures, and by the speed rating. You will find S and T rated tires with a max pressure of 44 or 51 psi, even though the speed rating test is performed at 35 psi.

Quote:
Originally Posted by RidingOnRailz View Post
……… Railz: Communists! ……
No, this is the height of capitalism. Making money off of selling information.

Quote:
Originally Posted by RidingOnRailz View Post
……… Railz: I will have to look at the size section on her sidewall again for that load index and speed letter, IE: '89T', 91V' or which ever hers might be. BTW on my Contour, the replacement tires did have the H embedded inside the size, and they were in fact H-rated tires.


So Capri: Would you consider it just happy coincidence that the formula I shared here worked for mine and my wife's cars and tires, yielding as it did a perfect split just above and below the equal recommended tire pressure on our door placards? Because we both really like how our cars drive after setting them to the values I derived.
No, the formula you shared is tantalizingly close. It just uses some faulty assumptions. The result it provides will be within shouting distance of what it would get if it hadn't made those assumptions.

Then there is the issue of what people like in the way of vehicle handling. When I was calling on Ford, I talked to the ride engineers - the ones the specify the spring, shock, and sway bar settings. Their job was to ignore their own preferences and delivery a product that met the goals as determined by the management team. But one of the items NOT on the agenda was tire pressure. That had been predetermined based on a procedure the company had loads of experience with.

However, that result of tire pressure, spring rate, sway bar size, and shock damping didn't always appeal to every driver and it is common for folks to tune their car by using a tire pressure different than what is on the vehicle tire placard. The danger is not using enough tire pressure - which increases the risk of a structural tire failure - or using too much that wet traction is significantly reduced.
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Old 07-01-2019, 10:22 PM   #24
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Cool Re: For Anyone Who Owns a Digital Tire Gauge

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Originally Posted by CapriRacer View Post
That would seem to confirm that it is the 15" option that has the pressure split




Oh, if it were just that simple. The tire manufacturer is the one who decides what max pressure is to be imprinted and he is constrained to use those 3 pressures, and by the speed rating. You will find S and T rated tires with a max pressure of 44 or 51 psi, even though the speed rating test is performed at 35 psi.



No, this is the height of capitalism. Making money off of selling information.



No, the formula you shared is tantalizingly close. It just uses some faulty assumptions. The result it provides will be within shouting distance of what it would get if it hadn't made those assumptions.

Then there is the issue of what people like in the way of vehicle handling. When I was calling on Ford, I talked to the ride engineers - the ones the specify the spring, shock, and sway bar settings. Their job was to ignore their own preferences and delivery a product that met the goals as determined by the management team. But one of the items NOT on the agenda was tire pressure. That had been predetermined based on a procedure the company had loads of experience with.

However, that result of tire pressure, spring rate, sway bar size, and shock damping didn't always appeal to every driver and it is common for folks to tune their car by using a tire pressure different than what is on the vehicle tire placard. The danger is not using enough tire pressure - which increases the risk of a structural tire failure - or using too much that wet traction is significantly reduced.

So, as far as my friend's 2005 Subaru Legacy GT is concerned:

I checked the size listed on the GY Eagles currently on the vehicle: 'P215/45R17', load & speed index: 87W.

87 load index = 1,201 lbs.

Now here's the kicker:
Her door placard lists original tire size: '215/45ZR17'.

So her current tires are not of the original Z speed rating, but are W. Probably saved whoever had them put on at least $50 per tire! This is a 15 year old car, so the exact size listed on that door placard is probably no longer available.

My smaller Hyundai's OEM T-rated(lower speed rating than W or Z!) tires have a higher load index - 91(1,356lbs) - on a smaller car! As well as my wife's: H-Rated 91's. That likely explains why the formula worked for both our rides, and why it didn't, initially, with my friend's Legacy GT. It's got the wrong tires on it! LOL

I've already run the formula under alll the scenarios, several posts above, so I'm not going to repeat here, but I do know that I got more reasonable results for that particular Subaru with the correct load & speed rated tires plugged into it.

Last edited by RidingOnRailz; 07-02-2019 at 10:16 AM.
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Old 07-06-2019, 01:35 PM   #25
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Cool Re: For Anyone Who Owns a Digital Tire Gauge

Quote:
Originally Posted by CapriRacer View Post
That would seem to confirm that it is the 15" option that has the pressure split




Oh, if it were just that simple. The tire manufacturer is the one who decides what max pressure is to be imprinted and he is constrained to use those 3 pressures, and by the speed rating. You will find S and T rated tires with a max pressure of 44 or 51 psi, even though the speed rating test is performed at 35 psi.



No, this is the height of capitalism. Making money off of selling information.



No, the formula you shared is tantalizingly close. It just uses some faulty assumptions. The result it provides will be within shouting distance of what it would get if it hadn't made those assumptions.

Then there is the issue of what people like in the way of vehicle handling. When I was calling on Ford, I talked to the ride engineers - the ones the specify the spring, shock, and sway bar settings. Their job was to ignore their own preferences and delivery a product that met the goals as determined by the management team. But one of the items NOT on the agenda was tire pressure. That had been predetermined based on a procedure the company had loads of experience with.

However, that result of tire pressure, spring rate, sway bar size, and shock damping didn't always appeal to every driver and it is common for folks to tune their car by using a tire pressure different than what is on the vehicle tire placard. The danger is not using enough tire pressure - which increases the risk of a structural tire failure - or using too much that wet traction is significantly reduced.
ZZZZZZ....zzzzzz-snorrre..zzz- Whoa, WAIT - Oh!

Where am I, what day is.. Oh!, how was your Fourth?


We just had our off-Site storage guy come for a pick up with his 2019 Nissan NV150 van.

Tires: LT245/70R17E, Max load 3000lbs, 2,755lbs 'dual' - assuming that means four per axle? Max cold pressure on tire 80psi. These tires match the B-pillar placard, of course probably OEM since the thing is less than one year old.

Anywho, ran my trusty-rusty formula for this van in its four-tire config., and got the following:

B-Pillar sticker on the van recommends: F 50psi, R 80psi, cold.

My calculations: F 51psi, R 78psi.

Tantalizing!


Do you know what that means, Capri?...

Last edited by RidingOnRailz; 07-07-2019 at 07:18 PM.
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Old 07-08-2019, 09:06 AM   #26
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Cool Re: For Anyone Who Owns a Digital Tire Gauge

Quote:
Originally Posted by RidingOnRailz View Post
ZZZZZZ....zzzzzz-snorrre..zzz- Whoa, WAIT - Oh!

Where am I, what day is.. Oh!, how was your Fourth?


We just had our off-Site storage guy come for a pick up with his 2019 Nissan NV150 van.

Tires: LT245/70R17E, Max load 3000lbs, 2,755lbs 'dual' - assuming that means four per axle? Max cold pressure on tire 80psi. These tires match the B-pillar placard, of course probably OEM since the thing is less than one year old.

Anywho, ran my trusty-rusty formula for this van in its four-tire config., and got the following:

B-Pillar sticker on the van recommends: F 50psi, R 80psi, cold.

My calculations: F 51psi, R 78psi.

Tantalizing!


Do you know what that means, Capri?...

It means that that 2005 Subaru is one WEIRD car, that's what it means, lol!

Even with specs for the correct OEM tires, my formula still returns relatively high recommendations for that Legacy GT - low 40s-psi vs low 30s on the B-pillar sticker.

It's the only car I've used the formula for that has returned such 'out-there' results.

I suspect it has something to do with the factory low-profile tires. Vehicles with wider, lower-profile tires on larger rims more often than not are speced with higher recommended cold pressures than vehicles with skinny high-profiles.
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