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Tire pressure for up-sized wheels

samd1351

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Maybe I'm missing something, and please in no way take the following as any kind of snary smartass response. I thought that there was a thread a while back about tire psi, and that no matter the size (generally speaking) you should run the psi as stated in the door panel, as that is what the car needs for proper support. The psi on the side of the tire is just a guide, with a max amount. At least that is what I recall from a previous thread. I know that there was some good info there, and if I can find a link to it I'll post it.

I read quite a bit of that thread as I assumed (incorrectly, at least according to that post) that you should always go with what is on the tire.
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Maybe I'm missing something, and please in no way take the following as any kind of snary smartass response. I thought that there was a thread a while back about tire psi, and that no matter the size (generally speaking) you should run the psi as stated in the door panel, as that is what the car needs for proper support. The psi on the side of the tire is just a guide, with a max amount. At least that is what I recall from a previous thread. I know that there was some good info there, and if I can find a link to it I'll post it.

I read quite a bit of that thread as I assumed (incorrectly, at least according to that post) that you should always go with what is on the tire.

Thanks for the tip. I checked the thread and it says more or less the same thing as I do; Wider tire - less pressure.

However, the pressure that is indicated on the tire is the maximum pressure for the tire just as the SWL is the maximum allowed load on a chain hoist. It has nothing to do with the pressure you run on the car, so that you can ignore (as long as you stay below it :cwl:)

Also, the pressure and the load rating is not the same thing. The load rating is how much load the tire can take in weight at a specific pressure as you can see in the table above. A tire with load rating 96 can ta 1415 lbs at 32 PSI but a tire with load rating 101 can handle 1647 lbs. It is a more "rigid" tire that can take a heavier car. If you put a tire with 80 load rating on a car that requires 107 and fill both with 32 PSI, the 80 will fail.

You want the pressure to correspond to the actual weight of the car at the tires specific load rating, and according to the table above (assuming that the load on the front wheels are 1415 based on the 32 PSI specified by Ford for the 96-tires) that translates to (as close as possible) 27 PSI on the front tires with 101 load rating.

You can drive a tire rated for X load at Y pressure, but it will then be under inflated and wear on the edges. You can also use the same tire with Z pressure and it will be over inflated and wear in the middle. Thats what the table above shows - i assume.

Spontaneously it feels a bit low with 26 PSI but that is 85% of 32 PSI and it wouldn't surprise me that my 305/35 tire has 15% larger volume than the stock 275/40, thus at that pressure carries the same load as can be read from the table above.

The table I picked is a standard table, so it might not be 100% correct for the PZeros even though tires are standardized. They all behave differently and that's why I started the thread as people might both know more than I do (not very hard) or have experience from the same setup and actually know what works.
If you tweak the pressure you can affect under/oversteering, wear, traction etc, so a lot of things matter even though there are a standard table.

Imagine a book shelf. If you put a book (pressure) on a shelf (tire) that is 4 ft long it will flex a little. Put the same book on a shelf that is 12 ft long and it will flex more. Imagine you want it to be straight as you want with a tire. You will then have to reduce the weight of the book (pressure) on the longer shelf (tire) to keep it plane (assuming the short shelf was straight with the original book).

I'm not trying to be a smart-ass, just want to say how I'm reasoning here :)
 

samd1351

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Thanks for the tip. I checked the thread and it says more or less the same thing as I do; Wider tire - less pressure.

However, the pressure that is indicated on the tire is the maximum pressure for the tire just as the SWL is the maximum allowed load on a chain hoist. It has nothing to do with the pressure you run on the car, so that you can ignore (as long as you stay below it :cwl:)

Also, the pressure and the load rating is not the same thing. The load rating is how much load the tire can take in weight at a specific pressure as you can see in the table above. A tire with load rating 96 can ta 1415 lbs at 32 PSI but a tire with load rating 101 can handle 1647 lbs. It is a more "rigid" tire that can take a heavier car. If you put a tire with 80 load rating on a car that requires 107 and fill both with 32 PSI, the 80 will fail.

You want the pressure to correspond to the actual weight of the car at the tires specific load rating, and according to the table above (assuming that the load on the front wheels are 1415 based on the 32 PSI specified by Ford for the 96-tires) that translates to (as close as possible) 27 PSI on the front tires with 101 load rating.

You can drive a tire rated for X load at Y pressure, but it will then be under inflated and wear on the edges. You can also use the same tire with Z pressure and it will be over inflated and wear in the middle. Thats what the table above shows - i assume.

Spontaneously it feels a bit low with 26 PSI but that is 85% of 32 PSI and it wouldn't surprise me that my 305/35 tire has 15% larger volume than the stock 275/40, thus at that pressure carries the same load as can be read from the table above.

The table I picked is a standard table, so it might not be 100% correct for the PZeros even though tires are standardized. They all behave differently and that's why I started the thread as people might both know more than I do (not very hard) or have experience from the same setup and actually know what works.
If you tweak the pressure you can affect under/oversteering, wear, traction etc, so a lot of things matter even though there are a standard table.

Imagine a book shelf. If you put a book (pressure) on a shelf (tire) that is 4 ft long it will flex a little. Put the same book on a shelf that is 12 ft long and it will flex more. Imagine you want it to be straight as you want with a tire. You will then have to reduce the weight of the book (pressure) on the longer shelf (tire) to keep it plane (assuming the short shelf was straight with the original book).

I'm not trying to be a smart-ass, just want to say how I'm reasoning here :)

Ok, so do I have this right?. So, if you have two tires, of different sizes, but with the same load rating, I can run the same psi. BUT, if I have two tires of the same size, but with different load ratings, then I need to run different psi. Correct? AND if I have two tires, of different sizes and with different load ratings, then I'm going to end up in the wall in turn 4!

If I have the above 2/3rds correct, I've learned something for today and now I can go home! If not, I'll hang my head low with shame.
 
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Ok, so do I have this right?. So, if you have two tires, of different sizes, but with the same load rating, I can run the same psi. BUT, if I have two tires of the same size, but with different load ratings, then I need to run different psi. Correct? AND if I have two tires, of different sizes and with different load ratings, then I'm going to end up in the wall in turn 4!

If I have the above 2/3rds correct, I've learned something for today and now I can go home! If not, I'll hang my head low with shame.
You are correct sir!

The last scenario is the one I'm facing and that I wanted to double check as I interpret it as follows:
The physical weight on the tire (the load or book in my bookshelf example) is the relevant thing.
To support that weight correctly you need 32 PSI in a 96Y tire but with a 102Y tire you need only 26 PSI. If you inflate it with 32 PSI it will be over inflated (for that load/weight) and wear in the center and give a smaller contact surface to the ground.

Compare it with.... A balloon vs a football.
Fill both with 10 PSI and put a certain weight on them. The balloon will pop but the football will take the load. Same PSI, same size, different load rating.
 

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im going to assume this is cold pressure.. or at heat up ? only asking cause this has me rethinking my pressures now lol, im at 102w, 1874 load front and 107y, 2145 for load ..rear. so that would be 36psi on each tire.. im just wondering if thats at cold pressure
 

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im going to assume this is cold pressure.. or at heat up ? only asking cause this has me rethinking my pressures now lol, im at 102w, 1874 load front and 107y, 2145 for load ..rear. so that would be 36psi on each tire.. im just wondering if thats at cold pressure
What wheels and tires? There’s no way you should be at 36 cold unless you’re running something that resembles a spare donut.
 

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As fas as I can tell, Ford recommends 32 psi front and rear for -all- of the OEM wheels and tire sizes across the line. Don’t ask me why, but this indicates to me that 32 psi cold is at least close to the right number in most of these situations.

235/50R18 - all-seasons 32 psi
255/40R19 - all seasons 32 psi
255/40R19 - summers 32 psi
275/40R19 rear tire - 32 psi
265/35R20 - 20” premium wheels 32 psi
305/35/R19 GT350 - 32 psi.

If you’re trying to help things by running 26psi or 36psi, it won’t. Try 32 cold unless you have a evidence to tweak it. Chances are the tire wear will be close enough at 32 you won’t need to.
 

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I figured. Cold pressure. Atm i was at 34 cold. But they been climbing to 37 to 38 at full warm-up. So. I'm going to knock them down to 32 tomm. Morning.. I'm running cooper rs3.s. 275.35.20. Front 305.35. Rear..
 

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I run my 275/305 Nitto G2s at 32 cold. Usually warm up to 34. My stock 19s( 255/40) roll out of the garage at 30 psi cold and warm up to 32. I think I am going to add a tad more air to my stock wheels( new tires DWS06's) . Maybe that will help with my 13.5 mpg around town and the softer sidewall.
 

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I've got my wheels fitted (finally) and got them filled with 2.9 bar to start with just to be on the safe side.
Rock hard and noisy... Today I decreased to 2.3 front and 2.4 rear and from what I can see on the contact surface they are more or less where they should be. Can perhaps deflate .1 on the rear to get stock pressure there too I believe. My camber is -2.0 and -1.8 in the rear since the shop didn't know how to adjust it, so I might buy Steeda camber adjustments to get them to approx -1.5 which should save some tires too.

Tires are 275/35R20 and 305/35R20 Pirelli Zeros. Front are XL and should acc to table have 28/29 PSI and rear approx 30 but since the front are a bit stretched I think they could need a tad higher, so for me, around 32 PSI might be the sweet spot COLD.
 

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IMO you have things backwards. Following your thoughts the higher load rated tire should have less pressure as it’s got more surface area, and stronger strength. But to play along with this you would then have the front narrower/shorter tires even higher yet due to the weight bias of the car being nose heavy big time. For tire wear the fronts should have even more yet due to the caster when turning wearing out the sides before the centers.

Does Ford state different pressures for gt350 or PP cars with staggered wheels and tires? Sure don’t. Does Ford state different pressures for all the possible factory sizes? Sure don’t.

Load ratings, pressures, sizes, and all this jargain. Put 32 in them cold and enjoy it.
 
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robwlf

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i just did 32 cold and im leaving it at that , if anything i might lower the back to 30 since there a 107y load rating
-2.0 camber and -1.8 is fine long as your tow is good

i run -1.8 front and -1.6 rear back is factory toe settings and the front has a little toe in .. my guy said he did the gt350 alignment settings and did some minor tweeks
 
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IMO you have things backwards. Following your thoughts the higher load rated tire should have less pressure as it’s got more surface area, and stronger strength. But to play along with this you would then have the front narrower/shorter tires even higher yet due to the weight bias of the car being nose heavy big time. For tire wear the fronts should have even more yet due to the caster when turning wearing out the sides before the centers.

Does Ford state different pressures for gt350 or PP cars with staggered wheels and tires? Sure don’t. Does Ford state different pressures for all the possible factory sizes? Sure don’t.

Load ratings, pressures, sizes, and all this jargain. Put 32 in them cold and enjoy it.
The load index is about the side wall and not the size of the wheel. A small tire with higher load can at a set pressure hold the same load as a larger tire with less load index and higher pressure.

With that said, I'm running 2,4 bar rear and 2,3 / 32 PSI (stock) front at it seems fine, almost a bit much (which would actually match what i've got from the tables.) Haven't experienced any loss of grip from the rear on accelerations so at least I've got more rubber in contact with the ground than before!
 

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Buy a tread depth gauge, check your tire tread depth regularly across the width and adjust pressure accordingly.

275/35/20 F=30 psi, 305/35/20 R=27psi seem to be the normal setting for these tires on my Mustang GT. G-Force comp-2 A/S BF Goodrich.

I will adjust the psi, up or down, depending on tread depth wear across the width of the tire. I have gone as low as 29psi on the fronts and 26psi on the rears to get the tread depth to equalize across the width.

These tires have 10k miles on them and the tread depth across the width of the tire are within .5mm of each other.

Also by doing this you can tell quickly if your alignment is correct/incorrect.

This seems to work for me, I'm no expert.
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