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Originally Posted By: OVERKILL

Good on you for ignoring his foolish advice
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The tire manufacturers have no idea what vehicle the tire is going on, subsequently, they also have no idea what the appropriate pressure is for that tire on a specific vehicle of a given weight, weight balance.....etc. They provide you with what the maximum inflation pressure is for that tire, which should be well above the pressure range specified by the manufacturers using that tire in their applications. This number is put there so that people who might be improperly using that tire in an application that doesn't call for it don't inflate it past that number and end up wearing it in their face, getting seriously injured or killed. If you've got an application that calls for 80psi on the placard and the tire you've got fitted has a max of 50 listed on the sidewall, you know the tire is not right for the application
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Considering how harsh the ride was in the Neon I was driving at the time under OE recommendations there was no way I could stand driving at 40psi+.

I recently added a 5th new tire for matching spare to my almost-new tires on the Mountaineer so I can do a 5-tire rotation (Because AWD) and was kinda surprised to see that Wal-Mart had inflated that tire to nearly 45psi as well. Door tag calls for 35. I guess I shouldn't be surprised though, as they only had the wheel and no door tag to refer to.
 
Originally Posted By: mattwithcats
You only have three numbers to work with,
the pressure listed on the door,
the max pressure on the OEM tire,
and the max pressure on the new tire...

Anyone got a better formula?


Yes, I do have a better formula.

I took the time, so here it is in a nutshell:

-Tires carry the load of your vehicle.
-Vehicle manufacturers specify a tire size and a tire pressure to acheive a safe load carrying limit.
-When tires of a different size are used, it is important to match the manufacturer's recommended load carrying limit as closely as possible without going below it.

As best I can determine from the information you have provided so far:

-Your car is a 2002 Saturn SL2
-Stock tires were 185/65R15
-Door placard recommends 29psi for the 185/65R15 tires
-You purchased new 205/60R15 tires

Here's the formula to determine what tire pressure you should be running in your new tires to safely manage the load of your car:

Stock 185/65R15 tires @ 29psi = 1,069 pounds Load Limit (load carrying capacity)

New 205/60R15 tires @ 26psi = 1,124 pounds Load Limit

These are industry standard values from a published industry source (2014 Yearbook of The Tire and Rim Association, Inc.)

So ... If the placard on your door says you should be running 29psi in your stock 185/65R15 tires, then you should be running 26psi in your new 205/60R15 tires.

It makes no difference what the Max Pressure number is on the sidewall of the tires.

It makes no difference what the speed rating is for the tires.

It's not rocket science ... just good engineering science.

HTH
 
Originally Posted By: mattwithcats
Bought two new tires this weekend...

For $100 out the door, two Hercules Tour 4.0 Plus, 205/60R15 91H...
http://www.herculestire.com/tire-gallery/passenger-and-light-truck/touring/tour-40-plus/

These are "H" rated, so the pressure on the door must be adjusted...
(the OEM's were "P" speed)
29 PSI recommended pressure on the door sticker, 35 PSI max on the OEM rubber...
Hence, 29 / 35 equal .83 inflation percentage...
These are 44 PSI max, so 44 times .83 equals 37 PSI adjusted...


The 44 psi max of your new tires is the maximum allowed cold pressure ( when inside tire temp = outside tire temp and about 65 /68 degr F.

This is what they only give on Standard Load tires and XL/reinforced/Extraload tires after about the year 2000.
The pressure needed for the maximum load up to 160km/99m/h is for Standard load in American system always 35 psi and XL// 41 psi. European system 36/42 with exeptions allowed.
This pressure is called maxloadpressure/ reference-pressure/ but will call it AT-pressure furtheron, because on LT tires they only give AT-pressure like this "maximum load xxxxlbs AT yyy psi( cold) "

So your new tires are probably also SL so AT pressure 35 psi .
So if your new tires have same maximum load or loadindex, you can keep same pressure.
Difference is speedcode , old tires P , where calculated in their maximum load for 150km/92m?/h and H speedrated with max speed of 210km/113m?/h are calculated in maxload for 160km/99m/h and for higher speed the AT-pressure has to be highened up with a system depending on speedcode.
Wil give a picture I made of the official system used in Europe of AT pressure highening up for the different speedcodes and maxload lowering 30 km/h below maximum speed for V/W/Y tires.
Also to make my answer be more noticed in this already 5 pages since yesterday.

referencepressurePtoY.jpg
 
Your previous tires were not "P" speed rated. The "P" you see is for P metric. Tire probably a S or T rated tire

also many vehicles have a air pressure sticker on the inside of the gas fill door that maybe a update/ different pressure from original pressure listed on the car door.post or owners manual
 
Now reading post above I realise that P stands for Passenger tire, if its given before the sises.
This is the American system and AT pressure is then 35 psi and if sises are the same as new they have 1 loadindex step lower then EURometric tires.
Then P205/60R15 is given and sometimes 90 S or T as tenderloin mentioned.
Then this maximum load or loadindex is calculated also for 160km/99m/h.
Its yust what the systems have decided to use as AT-pressure to calculate the maximum load for, that 1 psi gives 1 LI step more maximum load, but to laws of nature you can call them the same.

I googled the new sises and found XL version with 95 Loadindex so your new tires are SL with AT pressure of 36 psi because no P before the sises so EUR Eurometric tires.
 
Originally Posted By: mattwithcats
You only have three numbers to work with,
the pressure listed on the door,
the max pressure on the OEM tire,
and the max pressure on the new tire...

Anyone got a better formula?


Two of those numbers have nothing to do with what pressure you should run. They are simply engineering limits for that tire, which have nothing to do with your car.. so, yeah there is a better formula...the tire pressure listed on the door. Adjust as necessary, don't exceed the number on the tire.

You've derived a formula based on completely false premises. It makes no sense.

Did you read anything that CapriRacer linked? Barry's Tire a Tech is a great site.

Learn from the tire engineer.

Or make stuff up out of thin air.

Your call.
 
Originally Posted By: mattwithcats
So if I read this right,

1069 divided by 1124 equals .95

,95 times 29 PSI equals 28, am I correct?

So a load range 95 tire would be
1069 / 1521 = .70

.70 X 29 PSi = 20.1 PSi.

I repeat my question, Do you want to drive around with 20 PSI in your tires?

http://www.tires-easy.com/winter/nokian-wrg3/205-60-R15/tirecode/NOKI0881520560HXL

http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=35


I am glad you recognize that 20 psi is the wrong answer, but it is obvious you don't know why it is wrong. You do know about tire load tables, right? Why aren't you using them?
 
Originally Posted By: mattwithcats
OK, original rubber was a "P" speed tire,
with 35 PSI max...

Rear tires are an "H" with 51 PSI max,
and front are "H" with 44 PSI max...

Putting 30 PSI in is not enough,
that's 60% of max pressure for the rear...
In other words, it would be like putting
20 PSI in the OEM rubber...

Do you want to drive around with 20 PSI in your tires?


WHACK, WHACK, WHACK, WHACK! The original tires were NOT P-rated. They were S-rated. 30psi is plenty. Note that the WHEELS are almost certainly not rated for 50psi!

And some cars DO call for low tire pressures. My Colony Park called for 26psi in front. I recall my wife ran 24psi in the 31" tires on her Wrangler and 22 on her Samurai. I ran tires rated for 50psi at 30 on my Cherokee.
 
Originally Posted By: Quattro Pete
Originally Posted By: SHOZ
What about the hot tire pressures?

What about it?
When I did road racing the first thing one would do is use chalk on the sidewall to see how much the tire rolled over when cornering. Usually with a 35 psi tire I would bump it to 40 or 45 psi. At the end of a session the pressures would be over 50 psi.

And in the winter the hot tire pressure is lower (I would assume) than in the summer, when filling to the same cold tire pressure.
 
A couple of thoughts:

It's obvious Matt doesn't know what is going on - and he doesn't want to know. He can remain willfully ignorant if he wants. I'm done!

Second, racing tires is a whole different kettle of fish - particularly racing with street tires. On my race car with racing tires, I started at 22 psi and the pressure grew to 25 psi - pretty close to perfect! When I was using street tires (different car), I also used a lot of pressure and experienced a lot of pressure build up. That's because the street tires weren't designed for racing, and I was trying to compensate for that. Not to mention that the street tires couldn't get enough camber to sit flat to the pavement (the problem was in the car - a problem my race car didn't have!)
 
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Originally Posted By: SHOZ
What about the hot tire pressures?

I think that's why they spec cold tire pressure, which is not the same as operating pressure, which I'm sure is factored in. On my Corvette, it is easy to see on the TPMS, that they start out at 30psi on a cool morning, and later in the day when the air temp rises, and the tires get warm running down the highway, they can easily operate at 35psi. Next morning they will be back to 30psi. Tires have to operate in a wide range of temps, so just because you set them at 30, doesn't mean that's what they are rolling down the road at. If you set cold to max sidewall pressure, on a hot day they will be over that for sure.
 
Originally Posted By: Traction
Originally Posted By: SHOZ
What about the hot tire pressures?

I think that's why they spec cold tire pressure, which is not the same as operating pressure, which I'm sure is factored in. On my Corvette, it is easy to see on the TPMS, that they start out at 30psi on a cool morning, and later in the day when the air temp rises, and the tires get warm running down the highway, they can easily operate at 35psi. Next morning they will be back to 30psi. Tires have to operate in a wide range of temps, so just because you set them at 30, doesn't mean that's what they are rolling down the road at. If you set cold to max sidewall pressure, on a hot day they will be over that for sure.
But the cold temps vary. -20f to 80f are the range of cold temps for me where I live. I was always under the impression max pressure does not take into account the hot tire temp.
 
Originally Posted By: SHOZ
But the cold temps vary. -20f to 80f are the range of cold temps for me where I live.

That's why you are supposed to adjust your cold inflation pressure seasonally. If you set your cold pressure to the value listed on vehicle placard during summer when it's 80F outside, obviously it'll be at different PSI when you measure it again cold at -20F. So you need to set it again to the value listed on vehicle placard when the tires are cold at that point. Kind of self explanatory, but I just wanted to make sure we were all on the same page.

Quote:

I was always under the impression max pressure does not take into account the hot tire temp.

The value listed on the tire is max COLD inflation pressure that the tire was designed to safely handle. The actual (warmed up) pressure will be a few PSI higher once you start driving on the tire, and that's OK. The tire manufacturer already took that into account.
 
Originally Posted By: jadatis
Now reading post above I realise that P stands for Passenger tire, if its given before the sises.
This is the American system and AT pressure is then 35 psi and if sises are the same as new they have 1 loadindex step lower then EURometric tires.
Then P205/60R15 is given and sometimes 90 S or T as tenderloin mentioned.
Then this maximum load or loadindex is calculated also for 160km/99m/h.
Its yust what the systems have decided to use as AT-pressure to calculate the maximum load for, that 1 psi gives 1 LI step more maximum load, but to laws of nature you can call them the same.

I googled the new sises and found XL version with 95 Loadindex so your new tires are SL with AT pressure of 36 psi because no P before the sises so EUR Eurometric tires.


Did not read over the 5 pages of reactions, sorry, but reading back now I found you gave the old tire sises , so googled them to be 86 Loadindex for Standard Load P-tire(88 for Eurometric 92 for XL eurometric ).



This gives old tires maxload LI 86/ 530 kg /1170 lbs AT 35 psi ( up to 99m/h).
New tires LI 91/ 615kg/1356 lbs AT 36 psi ( up to 99m/h).
Your OEM car advice was 29 psi for the GAWR's given up to maximum technical car speed.

Now I am asuming , to make the calculation to be overseen , that your Tech max carspeed is 99m/h or lower.
EDIT: from here read for GAWR > half GAWR because 2 wheels on the axle.
First we calculate back wich GAWR they used to calculate this 29 psi.Originally they came to an answer of between 28 and 29 and rounded up to the first 1 psi.

First way is your system of part of AT- pressure = part of maximum load wich in fact is not that bad . 29/35 * 1170 lbs= 970 lbs GAWR of heavyest axle.
But also 28/35 * 1170 = 936 lbs , so GAWR is between 936 and 970 lbs.

Second way is the official European calculation.
(29/35)^0.8 * 1170 lbs= 1007 lbs GAWR max.
( 28/35)^0.8 * 1170 lbs= 979 lbs GAWR min.

Third way is how the official calculation was in America before 2006, then they chanched to the EUR calculation, but only for SL and XL//, and left LT to old calculation with ^0.7 ( power).
( 29/35)^0.5 * 1170 = 1065 lbs max
( 28/35)^0.5 * 1170 = 1047 lbs min

calculation 3 is probably used ( saw something like 1998 your car was build).
But you can check it on the car to find the GAWR ( Gross Axle Weight Rating) wich is mostly front and rear different, but carmakers often give same pressure advice F/R.

So I go from the maximum calculated GAWR to calculate the needed pressure for the new tires.

3 calculations again , but now , 1 yours logical calc, 2 official European calc( and after 2006 also official USA calc), and 3 my calculation wich gives higher pressure, and to my opinion comes closest to the ever to be constructed ideal one to laws of nature.
Calculation 1 = logical calc= part of maxload= part of AT-pressure.
(1065/1356)*36 psi= 28,27 psi max
(1047/1356)*36 psi= 27,79 psi min

Calc 2 Official Eur/USA calc nowadays.
( 1065/1356)^1.25 * 36 psi = 26.61 psi max.
( 1047/1356)^1.25 * 36 psi = 26.05 psi min.

Calc 3 My calc wich asumes a part of maxload to be carried by the construction of tire , for a SL tire about 6% wich is 81 lbs for the 1365 maxload of new tires, and a power of 1.052 only to give a curve in the line ( is 1/0,95 of power for the other way around calculation so load for pressure).
((1065-81)/(1356-81))^1.052 * 36 psi = 27.41 psi max
((1047-81)/(1356-81))^1.052 * 36 psi = 26,88 psi min.

So if the car maker did use the general system and used a max speed of car of 99m/h, you can use one psi lower so 28 psi ( max of my calculation) and your logical math is not that bad.
But only when you have the original weights ( GAWR's) , calculating used GAWR from adviced pressure gives to low GAWR and for that you calculate then to low advice pressure .

But check the GAWR's , those are used that time for calculating, and if your max tech carspeed is higher it has to be recalated with higher then AT-pressure.


Sorry for the long story, but I bolded the end-answer for you, and again , mind the buts and iffs.
 
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