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Old 13th June 2008, 09:16   #1
J Irwan
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Tire pressure formula for "plus sizing"

Luke, or any tire expert out there.

I tried to search on the web and tirerack website but couldnt find any definitive answer.

What prompt me this question, the other day I was reading the magazine article that highlight some of the changes from Porsche 996 to 997 which outline the 19" wheel setup. The article mentioned one of the advantage going from standard 18" to 19" beside more responsive steering , is something a long the line "the ride benefit from lower tires pressure".

So it seems that with plus sizing the recommend PSI is lower ?

For example if stock size is 245/40/18 (with a recommended pressure of 38psi) then if I were to upsize to 245/35/19 or 255/35/19 the recommended pressure for the new sizes would be lower.

Is there a formula to calculate the new tires pressure ?

Regardz,

J Irwan
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Old 13th June 2008, 21:50   #2
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First I've heard. You should still use the recommended tire pressure. The lower profile tires do not require a bump or reducton of pressure.
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Old 14th June 2008, 01:16   #3
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There is NO perfect tire pressure or formula. It is based on a number of things. Comfort, handling, noise, wear, brand of tire, etc.

You always start with the recommended pressure (on the driver's door jam) and adjust. I usually start about 3 pounds higher and go from there. Each driver has different requirements. And, the requirements differ based on how many people are in the car and how much "junk" is in the trunk ... Also, on speeds you like to maintain. Around town pressures should generally be lower than for running at 100 MPH or more for hours on end. You might even be wise to consider the weather conditions you are running in.

You have to check at least every two weeks as all tires lose air over time. About 1/2 pound a week, in my case.

You sould pick a number and reset your tire monitor. Understand that the monitor doesn't work over 95 MPH (per BMW).
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Old 14th June 2008, 02:50   #4
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You have to consider the aspect ratio of the new tires also, and tire pressure in relation to protecting the wheel from impacts.
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Old 14th June 2008, 05:03   #5
J Irwan
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Quote:
Originally Posted by jclyman View Post
There is NO perfect tire pressure or formula. It is based on a number of things. Comfort, handling, noise, wear, brand of tire, etc.

You always start with the recommended pressure (on the driver's door jam) and adjust. I usually start about 3 pounds higher and go from there. Each driver has different requirements. And, the requirements differ based on how many people are in the car and how much "junk" is in the trunk ... Also, on speeds you like to maintain. Around town pressures should generally be lower than for running at 100 MPH or more for hours on end. You might even be wise to consider the weather conditions you are running in.

You have to check at least every two weeks as all tires lose air over time. About 1/2 pound a week, in my case.

You sould pick a number and reset your tire monitor. Understand that the monitor doesn't work over 95 MPH (per BMW).


I understand that tire pressure is a preference that varies from one driver to another.

But I am more curious with a formula/guide when upsizing the wheels for a given tire pressure .

So for example: for the given size 245/40/18 and 274/35/18 I run both at 36 psi. (Again some people might run 38 psi, or even 41 psi all around for the same size).
The question is if I upsize my tires to 255/35/19 and 285/30/19 what should be my tire pressure...


So further research I came accross this formula:
[Original Tires Max Load Rating] * [Sticker's Recommended P.S.I.] / [Original Tires Max P.S.I.] * [New Tires Max P.S.I.] / [New Tires Max Load Rating].

So going from 245/40/18 & 275/35/18 to 255/35/19 & 285/30/19 translate to 0.9 formula . (this is specific based on the information for Dunlop 8080E and my Yokohama Advan Sport rating)
Meaning if I usually run 36 psi all around the my new-tire pressure would be 32.4 (36x0.9) psi.


Can anyone confirm this.

PS: I don't know if it makes sense but when I run my 19" tires (above size) with the 37psi the ride feel much firmer then when I have 36psi on my OEM 18". And I got more cushion on my 19" setup compare to my 18"


Regardz,

J Irwan
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Old 14th June 2008, 07:21   #6
ard
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Regarding the 'story' from Porsche marketing on the 996/997: I always get a laugh when I read about the 'improved performance' with upsizing to 19" from 18" wheels.... nobody seriously tracks with 19s.

Porsche changed to 19" only under pressure from US sales and marketing.

Back to the question. There is no formula- so many variables. It is a combination of the contact patch geometry and the sidewall construction that will drive the pressure decision.

What are you seeking to optimize with your pressure? Handling? Ride? tire wear? mileage?

Looking at your tire sizes, you are only increasing the width by 10mm, (245-->255 and 275-->285)... this is in the 4% range, of course this is width, and the contact patch is an area. Area will be more than 4%

I'd start at 2-3psi less.

A

PS You don't have more 'cushion'.... a 245/40 has a sidewall of 98/2mm or 49mm, whereas the 255/35 has 89.5/2 or 45mm.
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Old 14th June 2008, 09:00   #7
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Quote:
Originally Posted by ard View Post
Regarding the 'story' from Porsche marketing on the 996/997: I always get a laugh when I read about the 'improved performance' with upsizing to 19" from 18" wheels.... nobody seriously tracks with 19s.

Porsche changed to 19" only under pressure from US sales and marketing.

Back to the question. There is no formula- so many variables. It is a combination of the contact patch geometry and the sidewall construction that will drive the pressure decision.

What are you seeking to optimize with your pressure? Handling? Ride? tire wear? mileage?

Looking at your tire sizes, you are only increasing the width by 10mm, (245-->255 and 275-->285)... this is in the 4% range, of course this is width, and the contact patch is an area. Area will be more than 4%

I'd start at 2-3psi less.

A

PS You don't have more 'cushion'.... a 245/40 has a sidewall of 98/2mm or 49mm, whereas the 255/35 has 89.5/2 or 45mm.
I hear you about the marketing pitch, can't always believe everything they say.

The article mentioned about additional handling benefit: more traction from the larger tire size, in which I notice on my car going from 245/40/18 to 255/35/19 on the front (althought the size should only be 10mm different but when I let stand side by side between Dunlop 8080E 275/35/18 vs 255/35/19 (Yoko Advan Sport) the track width is almost as wide) the steering response is noticeably more responsive.






As I tried different pressure with the OEM, I found that 36 psi is within my liking (as far as ride, street handling and wear concern), hence I was looking into a formula(if one exist) to figure out similar setup on the upsizing.... Thats what I was looking for in the first place. A number to start with for the new tires.

36 psi (245/40/18) ~~ ? psi (255/35/19)


I am also curious why you say I should start with 2-3 psi less (in which is in the ballpark according the formula I found). I am just trying to understand your logic.


As far as cushion you're right the OEM 245/40/18 has more sidewall by ~ 4mm. For some reason I was comparing if I were upsizing to 245/35/19 vs 255/35/19


Regardz,

J Irwan
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Old 15th June 2008, 07:48   #8
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Quote:
Originally Posted by J Irwan View Post
I
I am also curious why you say I should start with 2-3 psi less (in which is in the ballpark according the formula I found). I am just trying to understand your logic.

Me too.



No, my logic was to use the different geometry of the contact patch that you would likely see with a wider and or 'longer' tire. (The contact patch is what you would see on the ground if looking from underneath on a piece of glass.)

It would be wider, since you are going from 245-255. It would be marginally longer, with a larger OD.

If the width is 4% wider, then the area is probably 10% larger (hence the better road feel... try 275s up front!) so if you have 10% more area, to support the same weight of the car, then you can use 10% less air.

Of course the contact patch can be reduced- if you put the same psi in the new tires, the car will 'ride higher' and the area of the patch will be exactly the same -in square inches- as it was with the old tire. Remember, if you have 900lbs on each wheel, and you have 30psi in the tires, your patch will be 30 sqin (30x30=900).

So my recommendation actually calculates a new psi that gives you the advantage of the larger tire and gives you a larger contact patch.

But, as with everything on the internet, who knows if this is 'right' or 'exact' or correct. It is a good place to start, and see how your wear, ride and handling progresses.

A
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Old 15th June 2008, 22:31   #9
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Courtesy of Sport Compact Car magazine:

Changing the tire size or going to a plus-size fitment will change the required inflation pressure somewhat. Within the range of optional OE tire sizes, the vehicle manufacturer's recommendations should be followed. For sizes outside the range, the tire manufacturer should be able to help. Yokohama, for instance, includes pressure recommendations for stock and plus-sizes, by vehicle, in its annual Fitment Guide, which your tire dealer should have a copy of. Typically, these are 0-3 psi higher for a Plus-one or Plus-two fitment. Also listed are maximum load ratings at maximum inflation pressure for each size of each tire Yokohama makes.
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Old 16th June 2008, 06:29   #10
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Quote:
Originally Posted by BKH View Post
Changing the tire size or going to a plus-size fitment will change the required inflation pressure somewhat. Within the range of optional OE tire sizes, the vehicle manufacturer's recommendations should be followed. For sizes outside the range, the tire manufacturer should be able to help. Yokohama, for instance, includes pressure recommendations for stock and plus-sizes, by vehicle, in its annual Fitment Guide, which your tire dealer should have a copy of. Typically, these are 0-3 psi higher for a Plus-one or Plus-two fitment. Also listed are maximum load ratings at maximum inflation pressure for each size of each tire Yokohama makes.

Interesting... .but the more I play around with the formula, the above statement is only correct on certain circumstance... (depending on the tires load rating)


Here is couple of spec of Bridgestone Potenza RE 050 that I got from Tirerack website.

225/45/17 Max load: 1323lbs Max psi: 51 diameter: 25"
225/40/18 Max load: 1235lbs Max psi: 51 diameter: 25.1"
255/40/18 Max load: 1521lbs Max psi: 51 diameter: 26.1"


If the OEM size were 225/40/17 and recommend psi is 35psi,

[Original Tires Max Load Rating] * [Sticker's Recommended P.S.I.] / [Original Tires Max P.S.I.] * [New Tires Max P.S.I.] / [New Tires Max Load Rating].



225/40/18: 1323 * 35 /51 * 51 / 1235 = 37.49 psi

255/40/18: 1323 * 35 /51 * 51 / 1521 = 30.44 psi


To me Its make more sense if the tire pressure is related to the tire load rating ?


Regardz,

J Irwan

Last edited by J Irwan; 16th June 2008 at 06:29.
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