Gross Axle Weight Percentage Tire Inflation Procedure

[RidingOnRailz]

I recently experimented with an inflation technique based on a percentage of the tire’s maximum inflation pressure as a percentage of that tire’s maximum load.

Using my 2010 Honda as an example:
GAWR-F(lbs.) = 2403
GAWR-Rr(lbs.) = 2061

To obtain the GAWR(lbs.) per tire, we divide the above GAWRs by 2:

GAWR-F per tire = 1201.5, rounded for simplicity to 1202
GAWR-Rr per tire = 1031

The tires on my car have a maximum load rating of 1477lbs at a maximum cold pressure of 51psi
The Tire & Load Info sticker on the driver’s door pillar recommends a cold pressure, in all four tires, of 32psi.

Next, we solve for percentages:
Front: 1202 is 81% of 1477
Rear: 1031 is 69% of 1477

Therefore:
81% of 51psi max cold inflation on our tires = 41psi(Front)
69% of 51psi max cold on our tires = 35psi(Rear) – Again, these figures are rounded for simplicity sake.

Soooo(!): I went out early one morning, best time to set cold tire pressures, and adjusted the inflation pressures on my 2010 Honda’s tires to: 41psi in front, and 35psi in the rear.
 

Driving impressions:
The car definitely had noticeably more ‘get-up-n-go’ than it did with the 32-33psi I typically ran since owning the car. The steering was a lot more responsive to even small inputs, yet the car still tracked *reasonably* straight at the higher pressures. Straight-line stability was almost what it was at the lower pressures, presumably because the difference between the front and rear cold pressures, as calculated above, was much higher than in previous experiments, where I would actually reduce rear cold pressure and increase front cold pressure, IE: 31 rear, 33 fronts.

During turns taken at slightly higher than typical speeds, the rear end still felt about as planted and less likely to come about on this front-wheel driver than than it did at the factory recommended tire pressures. Did this new setup have an effect on my fuel mileage? Probably not as much as the effect it had on the handling, acceleration, etc. Overall, handling felt a little crisper, for lack of a better term.

Ride:
The ride was distinctly firmer, and on warmer days, the tires transmitted significantly more road surfaces imperfections into the cabin and through the seating. Remember, I set the tires to the calculated higher pressures as I would normally set recommended cold pressures – the coolest part of the day, just before sunrise. On days where I go to work mid-shift(10-11am start), that means the tires probably have, in the fronts, 42-43psi, and in the rears, 36-37! At that point, the ride was positively rickshaw rattling rough! Leaving at 7am for my early shift, not so much, and relatively smooth. Though not as comfortable a ride as it would be with 32-33psi at all four corners.

Verdict:
You may use the above Gross Axle Weight Rating percentages to set cold tire pressures at your discretion, just be mindful of the more responsive steering and stiffer ride quality when doing so. And as always, if your calculations result in cold pressures equaling or exceeding maximum cold inflation pressures stamped on your tires – please – back off a bit.
For example, in my above case, if my tires had maximum inflation pressures of 44psi, I probably would shave 1-2psi off the calculated values, and put 34psi in the rear tires, and 39 in the fronts, leaving a reasonable safety margin.

For my wife's 2004 Toyota Corolla, the calculations result in the following cold tire pressures:

Factory: Fr. 30psi Rr. 30psi

Calculated based on Percent of GAWR to Maximum tire load:

Fr. 35psi Rr. 32psi

In both the Honda and Toyota examples, the cold pressures all around went up significantly, and a significant front-rear pressure difference also appeared.


For the tire experts here, please let me know what you think of my percentage-based procedure for setting cold tire pressures.

[CapriRacer]

First, the rated pressure of the tires is 35 psi. The MAX pressure is 51 psi. So if you want to do percentages, you need to use 35 psi NOT 51 psi.

Second, the load vs pressure curve is NOT proportional to the pressure. While it is close to a straight line (it actually isn't!), the relationship is such that even at zero psi, the tire can carry SOME load.

That's why they have tire load tables!.

I go into more detail here: http://barrystiretech.com/loadtables.html

But keep in mind that the vehicle manufacturer set up the balance of the car using the pressures listed - and a fact that you need to know is that the spring rate of a tire very closely follows the inflation pressure REGARDLESS of who makes the tire. It's a Physics thing!

Ergo, changing the front to rear pressure split will result in a change in the Oversteer/Understeer characteristics of the vehicle. I recommend doing such research very carefully.

Also, vehicle manufacturers conduct limit handling to see if the vehicle behaves in a predictable manner. This requires a set of tires to be severely damaged - that is totally unusable after the test. Obviously this requires resources that the average guy just doesn't have - both in terms of testing loaction as well as the volume of tires needed to conduct such tests.

So be very careful in that what may seem OK driving to the grocery store, may turn out to be very deadly in emergency maneuvers.

What might be interesting is to see what the vehicle manufacturer used as inflation pressure compared to the GVWR. My opinion is that EVERY tire pressure spec ought to be 115% of the rated pressure at the GVWR. That's one of the lessons from the Ford/Firestone situation back in the year 2000.

[RidingOnRailz]

Originally Posted by CapriRacer

First, the rated pressure of the tires is 35 psi. The MAX pressure is 51 psi. So
if you want to do percentages, you need to use 35 psi NOT 51 psi.

Second, the load vs pressure curve is NOT proportional to the pressure. While it is close to
a straight line (it actually isn't!), the relationship is such that even at zero psi, the tire can
carry SOME load.

That's why they have tire load tables!.

I go into more detail here: http://barrystiretech.com/loadtables.html

But keep in mind that the vehicle manufacturer set up the balance of the car using the
pressures listed - and a fact that you need to know is that the spring rate of a tire very
closely follows the inflation pressure REGARDLESS of who makes the tire. It's a Physics
thing!

Ergo, changing the front to rear pressure split will result in a change in the Oversteer/
Understeer characteristics of the vehicle. I recommend doing such research very carefully.

Also, vehicle manufacturers conduct limit handling to see if the vehicle behaves in a predictable
manner. This requires a set of tires to be severely damaged - that is totally unusable after
the test. Obviously this requires resources that the average guy just doesn't have - both in
terms of testing loaction as well as the volume of tires needed to conduct such tests.

So be very careful in that what may seem OK driving to the grocery store, may turn out to
be very deadly in emergency maneuvers.

What might be interesting is to see what the vehicle manufacturer used as inflation pressure
compared to the GVWR. My opinion is that EVERY tire pressure spec ought to be 115%
of the rated pressure at the GVWR. That's one of the lessons from the Ford/Firestone
situation back in the year 2000.

Thanks for your depth of insight, as expected.

But here is where I must refute:

Tires, for at least the last 40 years, have had stamped on them: “Max load XXXX at Max Cold XXpsi” And since the late 1990s, most passenger and LT tires have indicated on them a max cold inflation pressure of at least 44psi. Some, 51psi. Light Truck designations - 60psi and up.

The Pirelli Cinturato P7s on my Honda: “max cold 51psi”

The ContiProContacts on my wife’s Toyota: “max cold 51psi”

Mitsubishi recommends 39psi cold on the 2021 Mirage insurance rental car we got when my wife’s car was rear-ended and in the body shop. I'm sure the max tire sidewall figures on its tires were, at minimum, 44psi.

Are those figures on those tires typos? Or just “marketing”? And what is “rated” vs maximum, in this context?

Why use such an outdated 35psi as a baseline for calculating tire pressures in the manner that I did, above?

44-51psi max cold pressure leaves a lot of leeway for experimenting with ones cold tire pressures to determine effects on ride and drivability. And of course, I understand that those are maxima, not ideal inflation numbers.

[maxwedge]

All I will say here is when I campaigned a high horse power stock bodied car in drag racing competition I determined tire pressures pretty much based on either contact patch in a burnout or during a full throttle launch. Of course other factors could slightly alter that. 50 psi in a passenger car is ridiculous, talk about contact patch and ride. Wheel width, weight distribution are only some of the factors that would determine the cold pressures. For every day driving I would say the mfgrs recommended pressures should be pretty close.

[RidingOnRailz]

Originally Posted by maxwedge

All I will say here is when I campaigned a
high horse power stock bodied car in drag racing
competition I determined tire pressures pretty
much based on either contact patch in a burnout
or during a full throttle launch. Of course other
factors could slightly alter that. 50 psi in a passenger
car is ridiculous, talk about contact patch and ride.
Wheel width, weight distribution are only some of the
factors that would determine the cold pressures.
For every day driving I would say the mfgrs
recommended pressures should be pretty close.

Well, even with my procedure as outlined in my original post, my calculated alternate cold pressures, both my Honda and wifey’s Toyota, still ended up in the lower half of that window between vehicle mfgrs recommendation and the max pressure stamped on the tires.

Some drivers might compromise a bit higher, setting their cold pressures exactly halfway up between vehicle mfg recommendation and the max on the tire.

IE: That would result in 42psi cold all around on my Accord - Wayy too high for both comfort and for secure handling

[CapriRacer]

Originally Posted by RidingOnRailz

Thanks for your depth of insight, as expected.

But here is where I must refute:

Tires, for at least the last 40 years, have had stamped on them: “Max load XXXX at Max Cold XXpsi” And since the late 1990s, most passenger and LT tires have indicated on them a max cold inflation pressure of at least 44psi. Some, 51psi. Light Truck designations - 60psi and up.

The Pirelli Cinturato P7s on my Honda: “max cold 51psi”

The ContiProContacts on my wife’s Toyota: “max cold 51psi”

Mitsubishi recommends 39psi cold on the 2021 Mirage insurance rental car we got when my wife’s car was rear-ended and in the body shop. I'm sure the max tire sidewall figures on its tires were, at minimum, 44psi.

Are those figures on those tires typos? Or just “marketing”? And what is “rated” vs maximum, in this context?

Why use such an outdated 35psi as a baseline for calculating tire pressures in the manner that I did, above?

44-51psi max cold pressure leaves a lot of leeway for experimenting with ones cold tire pressures to determine effects on ride and drivability. And of course, I understand that those are maxima, not ideal inflation numbers.

If any of what you say is true, then there ought to be a published tire load table for STANDARD LOAD passenger car tires that shows something other than 35 psi as the rating point. So show me a link.

But if what I am saying is correct, you won't find such a table and all you'll find is 35 psi rated tire load tables.

BEWARE: Extra Load tires have their rating point at 41 psi.

BTW you need to read the link to my website because I answer that question in the (admittedly lengthy) discussion.

Once you've done all of that, I'll explain what is supposed to be written on the sidewall according to the US Federal regulations.

[RidingOnRailz]

Originally Posted by CapriRacer

If any of what you say is true, then there ought to be a published tire
load table for STANDARD LOAD passenger
car tires that shows something other than 35 psi as the
rating point. So show me a link.

But if what I am saying is correct, you won't find such a table
and all you'll find is 35 psi rated tire load tables.

BEWARE: Extra Load tires have their rating point at 41 psi.

BTW you need to read the link to my website because I answer that
question in the (admittedly lengthy) discussion.

Once you've done all of that, I'll explain what is supposed to
be written on the sidewall according to the US Federal regulations.

Very good, I will seek out that link about rating points, if you could specify which one will lead me to it - 'Tire Standardizing Organizations'? 'Load Tables'? 'Original Equipment Tires'?

As far as what is listed as the max. cold pressures on the tires in our household, I have no reason to lie to you. Both list a maximum cold inflation pressure of 51psi. "225-50R17" "94V" on the 2010 Honda Accord EX. The tire & load sticker specified 93 load V-speed, a combination here in 2022 as rare as hens teeth I'm afraid. I presume that would place both mine and my wifes tires in the 'Extra Load' category?

The Gross Axle Weight Ratings figures in my above calculations come right from the corresponding build sticker on my Honda.

The tires on my long gone college-mobile, a 1981 Buick Century, had a max cold inflation pressure on them stating 35psi. Which by the way was also GM's recommended cold pressure for Centurys and Regals of that era.

[CapriRacer]

Originally Posted by RidingOnRailz

Very good, I will seek out that link about rating points, if you could specify which one will lead me to it - 'Tire Standardizing Organizations'? 'Load Tables'? 'Original Equipment Tires'?

As far as what is listed as the max. cold pressures on the tires in our household, I have no reason to lie to you. Both list a maximum cold inflation pressure of 51psi. "225-50R17" "94V" on the 2010 Honda Accord EX. The tire & load sticker specified 93 load V-speed, a combination here in 2022 as rare as hens teeth I'm afraid. I presume that would place both mine and my wifes tires in the 'Extra Load' category?

The Gross Axle Weight Ratings figures in my above calculations come right from the corresponding build sticker on my Honda.

The tires on my long gone college-mobile, a 1981 Buick Century, had a max cold inflation pressure on them stating 35psi. Which by the way was also GM's recommended cold pressure for Centurys and Regals of that era.

First, I am going to do this in small increments so I apologize ahead of time for the number of posts and how long this is going to take.

Second, You need to be aware of some definitions:

Rated Load = the maximum load on the table

Rated Inflation = the pressure that corresponds to the Rated Load.

We tire engineers tend to use those terms. I'll explain that later.

Third, let's confine our discussion to Passenger Car tires. Light Trucks and other kinds of tires are done differently. I'll explain that later as well.

So I googled "Tire Load Tables" and came up with these:

Toyo: https://www.toyotires.com/media/3729...s_20200723.pdf

This one took a bit of digging deeper to get to the actual tables.

The thing to note here is that the rated pressure for Standard Load Passenger Car tires is either 35 or 36 psi - and that's because of the difference between English and metric measuring systems. In the metric system they use 250 kPa (kiloPascals) or 2.5 bar as the rating pressure. So depending on which system the tire was developed in, you'll get a slightly different rated pressure.

TirePressure.com: https://tirepressure.com/p-metric-ti...nflation-table

This one is for P metric tires. Here's one for Metric tires:

https://tirepressure.com/metric-tire...nflation-table

Nitto: https://www.nittotires.ca/sites/defa...ion_tables.pdf

This appears to be the same document as Toyo - except for formatting. I wonder if the same person did both.

Not sure about where this one is from: https://dcadprod.azureedge.net/b2c-e...344685c842.pdf

Anyway that is enough to get you started.

[RidingOnRailz]

Originally Posted by CapriRacer

First, I am going to do this in small increments so I apologize ahead of time for the number of posts and how long this is going to take.

Second, You need to be aware of some definitions:

Rated Load = the maximum load on the table

Rated Inflation = the pressure that corresponds to the Rated Load.

We tire engineers tend to use those terms. I'll explain that later.

Third, let's confine our discussion to Passenger Car tires. Light Trucks and other kinds of tires are done differently. I'll explain that later as well.

So I googled "Tire Load Tables" and came up with these:

Toyo: https://www.toyotires.com/media/3729...s_20200723.pdf

This one took a bit of digging deeper to get to the actual tables.

The thing to note here is that the rated pressure for Standard Load Passenger Car tires is either 35 or 36 psi - and that's because of the difference between English and metric measuring systems. In the metric system they use 250 kPa (kiloPascals) or 2.5 bar as the rating pressure. So depending on which system the tire was developed in, you'll get a slightly different rated pressure.

TirePressure.com: https://tirepressure.com/p-metric-ti...nflation-table

This one is for P metric tires. Here's one for Metric tires:

https://tirepressure.com/metric-tire...nflation-table

Nitto: https://www.nittotires.ca/sites/defa...ion_tables.pdf

This appears to be the same document as Toyo - except for formatting. I wonder if the same person did both.

Not sure about where this one is from: https://dcadprod.azureedge.net/b2c-e...344685c842.pdf

Anyway that is enough to get you started.

The Pirellis on my Honda have no 'P' in front of the size stamped on them.

So, I went to the metric tables, for size 225/50R17 93V:

https://tirepressure.com/225-50r17-tire-pressure

According to that table, at Honda's recommended 32psi, each tire can carry a maximum of 1366lbs.

Also, the standard load tire in that size can carry its maximum load, 1477lbs, at 36psi according to the table.

BUT: Stamped on the tires is: "Max Load 1477lbs at 51psi Max Pres."

So which, pray tell, is correct - the table, or the tire manufacturer?

And also: How can I determine what load type my tires are - SL(Standard) or XL(Extra/Extended)? Again, it is nearly impossible to find the 93V rated tire in the size specified on my car's sticker, twelve years after it was built, so that is why I have 94V rateds on it.

The table above, for my specific tires, goes up only to 42psi, for XL in my size

[CapriRacer]

Originally Posted by RidingOnRailz

.....
BUT: Stamped on the tires is: "Max Load 1477lbs at 51psi Max Pres."

So which, pray tell, is correct - the table, or the tire manufacturer? ......

The table is correct and allow me to take a roundabout journey to explain why.

The US regulations for tires says that the tire must be stamped with the maximum load and pressure. For many years, most tire manufacturers interpreted that to mean the max load and its corresponding pressure = rated load/rated pressure. This meant that the tire would say XXXX max load at YY pressure.

However, someone pointed out that the tire standards allow more pressure for H and higher speed rated tires - and the consumer likely would interpret the above to mean that YY was the max pressure. So most tire manufacturers adopted this: Max Load XXXX, Max Pressure YY.

But some got sloppy - and apparently Pirelli was one of those - and rewrote that as Max load XXXX at YY max pressure. This isn't exactly wrong, but it is inaccurate and misleading. (I suspect that when the max pressure thing was pointed out, some decided to just change the pressure writing on the existing molds and stupidly just copied that going forward.)

So that's why you will NOT find a table that lists 51 psi on it (or for that matter 44 psi!), because that is NOT the standard for load vs inflation pressure.

Originally Posted by RidingOnRailz

....... And also: How can I determine what load type my tires are - SL(Standard) or XL(Extra/Extended)? .....

It ought to say on the tire, but, again, some folks are sloppy and since it isn't required by regulation, it might not be there.

Originally Posted by RidingOnRailz

....... Again, it is nearly impossible to find the 93V rated tire in the size specified on my car's sticker .....

And the reason for that is that many folks think that it is NOT OK to use a lower Load Index than specified, but it is OK to use a higher one - even though the tires are identical, just specified to different standards.

Put another way, it is OK to use a P metric SL tire in place of a metric SL tire and vice versa even though the standards make it look like it is a lower rated tire. that's one of the things my web page on load tales explains.

[RidingOnRailz]

Originally Posted by CapriRacer

The table is correct and allow me to take a roundabout journey to explain why.

The US regulations for tires says that the tire must be stamped with the maximum load and pressure. For many years, most tire manufacturers interpreted that to mean the max load and its corresponding pressure = rated load/rated pressure. This meant that the tire would say XXXX max load at YY pressure.

However, someone pointed out that the tire standards allow more pressure for H and higher speed rated tires - and the consumer likely would interpret the above to mean that YY was the max pressure. So most tire manufacturers adopted this: Max Load XXXX, Max Pressure YY.

But some got sloppy - and apparently Pirelli was one of those - and rewrote that as Max load XXXX at YY max pressure. This isn't exactly wrong, but it is inaccurate and misleading. (I suspect that when the max pressure thing was pointed out, some decided to just change the pressure writing on the existing molds and stupidly just copied that going forward.)

So that's why you will NOT find a table that lists 51 psi on it (or for that matter 44 psi!), because that is NOT the standard for load vs inflation pressure.



It ought to say on the tire, but, again, some folks are sloppy and since it isn't required by regulation, it might not be there.



And the reason for that is that many folks think that it is NOT OK to use a lower Load Index than specified, but it is OK to use a higher one - even though the tires are identical, just specified to different standards.

Put another way, it is OK to use a P metric SL tire in place of a metric SL tire and vice versa even though the standards make it look like it is a lower rated tire. that's one of the things my web page on load tales explains.

So two questions:

1.) My 2010 Honda specified on its Tire and Load placard:

"P225/50R17 93V" for its tires.

At the time I wanted to replace the tires on the car when I bought it(2020), the above OEM size was no longer available(or not easily attainable) in that combination 93 load factor and V speed rating. And why would 93V become unavailable so soon after that generation of Accord come out in the first place?

The tires closest to 93V were 94V or higher, on the load side, up to 98V.

What ended up on the car were "225/50R17 94V" Pirelli P7s.

My question is, is it OK for these particular tires to be on my car? I mean, at or just a hair above the 32psi specified, the acceleration, handling and braking are, well, car-like! Secure and predictable in every aspect, including a few close calls which required quicker than usual turning or braking. I feel like I am always within all safety parameters with the Pirellis.

2.) Exactly what is the maximum safe cold inflation pressure on my Pirelli tires: 35, 36, 42psi, or something else? Not the "rated" max pressure, but theee max cold pressure?

Now you've made me think more about my tires, and tires in general, and I want to know more so I can share with the ignorant legions out there!

[CapriRacer]

Originally Posted by RidingOnRailz

So two questions:

1.) My 2010 Honda specified on its Tire and Load placard:

"P225/50R17 93V" for its tires.

At the time I wanted to replace the tires on the car when I bought it(2020), the above OEM size was no longer available(or not easily attainable) in that combination 93 load factor and V speed rating. And why would 93V become unavailable so soon after that generation of Accord come out in the first place? .....

To answer this, I am going to have to start in a different place.

P metric tires generally have lower load ratings because the rated pressure is lower - 35 psi vs 250 kPa (36.3 psi). The tires are identical, but because of the way they are rated, the numbers come out different.

The rule is to not apply tires with a lower load rating. So it just doesn't make sense for a tire manufacturer to produce a P metric tire in a given size when a metric tire will be rated at a higher load rating.

Further, if some vehicle manufacturer specifies an Extra Load tire, it generally doesn't make sense for a tire manufacturer to also produce a Standard Load version.

Originally Posted by RidingOnRailz

..... The tires closest to 93V were 94V or higher, on the load side, up to 98V.

What ended up on the car were "225/50R17 94V" Pirelli P7s.

My question is, is it OK for these particular tires to be on my car? I mean, at or just a hair above the 32psi specified, the acceleration, handling and braking are, well, car-like! Secure and predictable in every aspect, including a few close calls which required quicker than usual turning or braking. I feel like I am always within all safety parameters with the Pirellis. .....

In a lot of respects, what you are talking about as a difference (93 vs 94) is merely a matter of labeling. The Physics says there really isn't a difference here.

What would be different is how one tire manufacturer would design a tire vs how another one would. THOSE differences could be quite dramatic.

Originally Posted by RidingOnRailz

....... 2.) Exactly what is the maximum safe cold inflation pressure on my Pirelli tires: 35, 36, 42psi, or something else? Not the "rated" max pressure, but theee max cold pressure? .......

This is a hard question to answer.

First deviating much from the placard pressure is going to affect how the vehicle behaves. It isn't like there is a bright line between safe and unsafe.

Plus the tire itself will have an effect once you have established where that line is.

As a general rule, being near the placard pressure doesn't affect the difference is handling enough to worry about. Personally, I would never go below the placard pressure, but 3 to 5 psi over that pressure shouldn't make that much difference in the safety aspect of things. Yes, it will affect the ride and handling, etc., but that usually isn't what we think of when we talk " Tire Safety".

Did you notice I referenced the placard pressure and not anything written on the tire sidewall or on the tire load table? That's because it's the vehicle that is determining how the vehicle behaves and the tire is just part of that system, not an entity by itself. The safety aspect is about how the vehicle handles.

In other words, the tire pressure specification is very much like the sway bar diameter. The vehicle manufacturer specifies these things to get the vehicle to behave in a certain way. The reason we talk about tire pressure so much is because:

1) Tires leak
2) Tire pressure varies with ambient temperature
3) It's something that the average consumer can easily change. (Changing sway bars is much more difficult.)

Originally Posted by RidingOnRailz

......Now you've made me think more about my tires, and tires in general, and I want to know more so I can share with the ignorant legions out there!

When I first discovered the internet, I also discovered how much misinformation about tires there was out there. I made it my goal in life to reduce this as much as I can.

And one last thought: Have you considered emailing Pirelli to ask about providing a load table for your tire? Their response might be interesting especially if they contradict what is written on the sidewall (which is what I expect them to do.)

[RidingOnRailz]

Originally Posted by CapriRacer

To answer this, I am going to have to start in a different place.

P metric tires generally have lower load ratings because the rated pressure is lower - 35 psi vs 250 kPa (36.3 psi). The tires are identical, but because of the way they are rated, the numbers come out different.

The rule is to not apply tires with a lower load rating. So it just doesn't make sense for a tire manufacturer to produce a P metric tire in a given size when a metric tire will be rated at a higher load rating.

Further, if some vehicle manufacturer specifies an Extra Load tire, it generally doesn't make sense for a tire manufacturer to also produce a Standard Load version.



In a lot of respects, what you are talking about as a difference (93 vs 94) is merely a matter of labeling. The Physics says there really isn't a difference here.

What would be different is how one tire manufacturer would design a tire vs how another one would. THOSE differences could be quite dramatic.



This is a hard question to answer.

First deviating much from the placard pressure is going to affect how the vehicle behaves. It isn't like there is a bright line between safe and unsafe.

Plus the tire itself will have an effect once you have established where that line is.

As a general rule, being near the placard pressure doesn't affect the difference is handling enough to worry about. Personally, I would never go below the placard pressure, but 3 to 5 psi over that pressure shouldn't make that much difference in the safety aspect of things. Yes, it will affect the ride and handling, etc., but that usually isn't what we think of when we talk " Tire Safety".

Did you notice I referenced the placard pressure and not anything written on the tire sidewall or on the tire load table? That's because it's the vehicle that is determining how the vehicle behaves and the tire is just part of that system, not an entity by itself. The safety aspect is about how the vehicle handles.

In other words, the tire pressure specification is very much like the sway bar diameter. The vehicle manufacturer specifies these things to get the vehicle to behave in a certain way. The reason we talk about tire pressure so much is because:

1) Tires leak
2) Tire pressure varies with ambient temperature
3) It's something that the average consumer can easily change. (Changing sway bars is much more difficult.)



When I first discovered the internet, I also discovered how much misinformation about tires there was out there. I made it my goal in life to reduce this as much as I can.

And one last thought: Have you considered emailing Pirelli to ask about providing a load table for your tire? Their response might be interesting especially if they contradict what is written on the sidewall (which is what I expect them to do.)

Thank you so much! I am enlightened, and I hope to enlighten others. I know of many drivers, online and in person, and even some mechanics, who keep theirs and inflate their customers’ tires at or near the maximum cold pressure stamped on the tire(!) Heaven knows just how over-inflated their tires really are! 😮

On my wife’s 2004 Toyota, the door placard specifies load, size, and pressure for ‘P’- rated tires. She is on her second set of Conti ProContacts, which are metrics(no P stamped in front of the size). And as in the case of my 2010 Accord, the Toyota now requires one PSI extra pressure to maintain the OEM load at the pressure specified by Toyota.

So the Honda is now 33psi cold, and the Toyota, 31psi cold.

Both your explanations, and the tables you steered(ha ha!) me toward, underscore the importance of staying as close as possible to vehicle recommended cold pressures.

Those tables can also be useful in the case of verifying suspiciously low or high vehicle pressure recommendations - a la early Explorer! - and steer drivers of such vehicles toward maintaining tire pressures more in line with the load and handling capacities of such vehicles

[CapriRacer]

Originally Posted by RidingOnRailz

Thank you so much! I am enlightened, and I hope to enlighten others. I know of many drivers, online and in person, and even some mechanics, who keep theirs and inflate their customers’ tires at or near the maximum cold pressure stamped on the tire(!) Heaven knows just how over-inflated their tires really are! 😮

On my wife’s 2004 Toyota, the door placard specifies load, size, and pressure for ‘P’- rated tires. She is on her second set of Conti ProContacts, which are metrics(no P stamped in front of the size). And as in the case of my 2010 Accord, the Toyota now requires one PSI extra pressure to maintain the OEM load at the pressure specified by Toyota.

So the Honda is now 33psi cold, and the Toyota, 31psi cold.

Both your explanations, and the tables you steered(ha ha!) me toward, underscore the importance of staying as close as possible to vehicle recommended cold pressures.

Those tables can also be useful in the case of verifying suspiciously low or high vehicle pressure recommendations - a la early Explorer! - and steer drivers of such vehicles toward maintaining tire pressures more in line with the load and handling capacities of such vehicles

You seem to have a bit of (another!) misconception. There is NO difference between P metric and metric tires. The apparent differences is caused by the different rating systems, not because the tires are different.

In both systems, the formula used to calculate the load carrying capacity is trying to estimate the amount of deflection and keep it the same for every point on the table. Since this was done long before there were computers, there have to be some assumptions made to simplify the task. Since different people did these calculations, they made different assumptions - and therefore got slightly different results. the operative word here is SLIGHTLY.

Therefore, it doesn't matter if the vehicle placard specifies a P metric or a metric, the inflation pressure is the same regardless.

[RidingOnRailz]

Originally Posted by CapriRacer

You seem to have a bit of (another!) misconception. There is NO difference between P metric and metric tires. The apparent differences is caused by the different rating systems, not because the tires are different.

In both systems, the formula used to calculate the load carrying capacity is trying to estimate the amount of deflection and keep it the same for every point on the table. Since this was done long before there were computers, there have to be some assumptions made to simplify the task. Since different people did these calculations, they made different assumptions - and therefore got slightly different results. the operative word here is SLIGHTLY.

Therefore, it doesn't matter if the vehicle placard specifies a P metric or a metric, the inflation pressure is the same regardless.

Well, according to tirepressure . com’s old vs new tire calculator, when I put in “P225/50R17” and Standard Load for existing tire, and Metric 225/50R17, standard load for replacement tire, the converter calculated a cold pressure for the replacment tires 1psi higher than for the P-metric. 33 vs 32.

Their website, their calculator, not mine. Same happened for my wife’s tires. 31psi for metric vs 30psi for OEM P-metric. Indeed, a “slight difference”, as you yourself stated. But a difference, nonetheless.

Even in your examples on your own website, going from a P-metric tire to a Metric resulted in a higher tire pressure to carry the same load as before.

Grass is green and sky is blue. I report what I see and what I experience