What do u run for tire pressures?
Melting Slicks
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From: www.Z16.org North/West Pennsylvania
The best way, I have found, to determine the best pressure for non stock sizes is the masking tape method. Get the tires warmed up, put a piece of 2” masking tape across the tread on all four tires. Drive it for a couple miles, then pull over and check the tape. Worn in the middle only, you need to drop the pressure, worn on the edges only, you need to increase the pressure, nice even wear across the tread and you got it. 
Pro
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From: Columbia South Carolina
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St. Jude Donor '17
Riddle me this Mr Wizard. You have 8" rims 17 / 18's and you change 3 foot wide rims with whatever size tires you can imagine.
You are telling me I am suppose to still use the PSI rating that the manufacture recommended for the stock rims and tires?
What if this new rim and tire size requires a different amount of PSI in order to seal the bead on the rim?
What page in your physics book should I turn to?
Last edited by Got uid0; May 27, 2011 at 06:53 PM.
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St. Jude Donor '17
The best way, I have found, to determine the best pressure for non stock sizes is the masking tape method. Get the tires warmed up, put a piece of 2” masking tape across the tread on all four tires. Drive it for a couple miles, then pull over and check the tape. Worn in the middle only, you need to drop the pressure, worn on the edges only, you need to increase the pressure, nice even wear across the tread and you got it.
Drifting
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LOL so you took a physics class and now an expert mechanic
Riddle me this Mr Wizard. You have 8" rims 17 / 18's and you change 3 foot wide rims with whatever size tires you can imagine.
You are telling me I am suppose to still use the PSI rating that the manufacture recommended for the stock rims and tires?
What if this new rim and tire size requires a different amount of PSI in order to seal the bead on the rim?
What page in your physics book should I turn to?
Riddle me this Mr Wizard. You have 8" rims 17 / 18's and you change 3 foot wide rims with whatever size tires you can imagine.
You are telling me I am suppose to still use the PSI rating that the manufacture recommended for the stock rims and tires?
What if this new rim and tire size requires a different amount of PSI in order to seal the bead on the rim?
What page in your physics book should I turn to?
Tires will seal around the rim at 30PSI... Trust me, it doesn't take Mr. Wizard to know that...
And 30PSI is the same amount of pressure regardless of how much area you're pressurizing. I agree with ztheusa. You must have been absent on the day you're supposed to learn this stuff in Jr. High.
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From: This is not a Song, It's an Outburst: Or, The Establishment Blues; Sixto Diaz Rodriguez
28 cold on my 17/18's during the Summer when temps down here easily reach 98/100*. I've noticed running 28 cold, gets up to 34 once we're rolling. Winter time, I run 30 cold. Just my 2 cents. 
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I run 30 psi cold on mine.
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St. Jude Donor '17
Tires will seal around the rim at 30PSI... Trust me, it doesn't take Mr. Wizard to know that...
And 30PSI is the same amount of pressure regardless of how much area you're pressurizing. I agree with ztheusa. You must have been absent on the day you're supposed to learn this stuff in Jr. High.
A rule of thumb is for every 10° Fahrenheit change in air temperature, your tire's inflation pressure will change by about 1 psi (up with higher temperatures and down with lower).
Let me help make my point a bit more clear for you.
I have stock rims and put on larger tires on my stock rims. The larger tires sidewalls are bulging more than they should at 30psi and gas mileage is less.
I increased the PSI slightly for my new setup and it gave me better performance and MPG.
Do you understand my point? There is not an exact set PSI to use and it may vary depending on your application.
You might even want to use a different PSI at the track for a different performance
response but you will probably tell me i need to read a book.
http://www.tirerack.com/tires/tirete...e.jsp?techid=1
Disadvantages of Underinflation
An underinflated tire can't maintain its shape and becomes flatter than intended while in contact with the road. If a vehicle’s tires are underinflated by only 6 psi it could lead to tire failure. Additionally, the tire’s tread life could be reduced by as much as 25%. Lower inflation pressure will allow the tire to deflect (bend) more as it rolls. This will build up internal heat, increase rolling resistance and cause a reduction in fuel economy of up to 5%. You would experience a significant loss of steering precision and cornering stability. While 6 psi doesn’t seem excessively low, remember, it usually represents about 20% of the tire’s recommended pressure.
Disadvantages of Overinflation
An overinflated tire is stiff and unyielding and the size of its footprint in contact with the road is reduced. If a vehicle's tires are overinflated by 6 psi, they could be damaged more easily when running over potholes or debris in the road. Higher inflated tires cannot isolate road irregularities well, causing them to ride harsher. However, higher inflation pressures usually provide an improvement in steering response and cornering stability up to a point. This is why participants who use street tires in autocrosses, track events and road races run higher than normal inflation pressures. The pressure must be checked with a quality air gauge as the inflation pressure cannot be accurately estimated through visual inspection.
An underinflated tire can't maintain its shape and becomes flatter than intended while in contact with the road. If a vehicle’s tires are underinflated by only 6 psi it could lead to tire failure. Additionally, the tire’s tread life could be reduced by as much as 25%. Lower inflation pressure will allow the tire to deflect (bend) more as it rolls. This will build up internal heat, increase rolling resistance and cause a reduction in fuel economy of up to 5%. You would experience a significant loss of steering precision and cornering stability. While 6 psi doesn’t seem excessively low, remember, it usually represents about 20% of the tire’s recommended pressure.
Disadvantages of Overinflation
An overinflated tire is stiff and unyielding and the size of its footprint in contact with the road is reduced. If a vehicle's tires are overinflated by 6 psi, they could be damaged more easily when running over potholes or debris in the road. Higher inflated tires cannot isolate road irregularities well, causing them to ride harsher. However, higher inflation pressures usually provide an improvement in steering response and cornering stability up to a point. This is why participants who use street tires in autocrosses, track events and road races run higher than normal inflation pressures. The pressure must be checked with a quality air gauge as the inflation pressure cannot be accurately estimated through visual inspection.
Last edited by Got uid0; May 28, 2011 at 02:16 AM.
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St. Jude Donor '17
I guess they did not teach you this in physics
All tires sold in the US are required by regulation to indicate the maximum load carrying capacity and the maximum usable pressure - and most of the time those 2 items are not connected. This is governed by a regulation that is a bit vague and there are different interpretations as to what is required.
However, this is also controlled by tire standardizing organizations. While the tire standards are not exactly the same throughout the world, they are close enough that you can consider a particular "size" to carry the same load at the same pressure.
This is convenient (and purposely done this way) because on every vehicle sold in the US, there is a sticker - commonly called the placard - which lists the original tire size and the proper pressure for that size. This is also a government regulation and is determined by the vehicle manufacturer.
The derivation for the tire standard (and therefore that load that is imprinted on the sidewall) is a table - commonly referred to as the "load table". It is a somewhat ideal figure - smooth surfaces, moderate speeds, moderate temperatures, etc. - and recent events have pointed out the shortcoming of the load tables.
The vehicle manufacturers and the tire manufacturers both realized that they could not change the way this is done without completely scrapping the tire size methodology, so the vehicle manufacturers were encouraged to oversize the tire (which most of them were doing anyway, and recent events encouraged then to go further).
Vehicle manufacturers use a tire larger than the minimum - and that just is good engineering practice. However, the methodology to determine what they use is considered a trade secret. We tire engineers have developed a term called "Reserve Capacity" to help us get a grasp of what a particular vehicle manufacturer is doing.
The short answer is that you should use the vehicle placard for pressure if you are using the same tire size as listed on the placard. If you are resizing the tires, use a tire that has the same load carrying capacity as the original. The "Load Index" is an easy way to determine if the tires are the same capacity.
http://en.allexperts.com/q/Tires-235...x-load-psi.htm
However, this is also controlled by tire standardizing organizations. While the tire standards are not exactly the same throughout the world, they are close enough that you can consider a particular "size" to carry the same load at the same pressure.
This is convenient (and purposely done this way) because on every vehicle sold in the US, there is a sticker - commonly called the placard - which lists the original tire size and the proper pressure for that size. This is also a government regulation and is determined by the vehicle manufacturer.
The derivation for the tire standard (and therefore that load that is imprinted on the sidewall) is a table - commonly referred to as the "load table". It is a somewhat ideal figure - smooth surfaces, moderate speeds, moderate temperatures, etc. - and recent events have pointed out the shortcoming of the load tables.
The vehicle manufacturers and the tire manufacturers both realized that they could not change the way this is done without completely scrapping the tire size methodology, so the vehicle manufacturers were encouraged to oversize the tire (which most of them were doing anyway, and recent events encouraged then to go further).
Vehicle manufacturers use a tire larger than the minimum - and that just is good engineering practice. However, the methodology to determine what they use is considered a trade secret. We tire engineers have developed a term called "Reserve Capacity" to help us get a grasp of what a particular vehicle manufacturer is doing.
The short answer is that you should use the vehicle placard for pressure if you are using the same tire size as listed on the placard. If you are resizing the tires, use a tire that has the same load carrying capacity as the original. The "Load Index" is an easy way to determine if the tires are the same capacity.
http://en.allexperts.com/q/Tires-235...x-load-psi.htm
Thread Starter
Le Mans Master
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From: Conn.
St. Jude Donor '21
Yup..been here long enough is right...you ever think that maybe this question is a result of JUST purchasing new tires and wheels which are not stock? DUH........
Melting Slicks
Joined: Apr 2003
Posts: 2,150
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From: NS
The placard pressure is not exact but a very good starting point. Adjust pressure based on wear over a period of time. The masking tape idea above is not a bad idea, gives you a "rough" idea of the tire's footprint on the road under normal conditions but long term tire wear pattern is the ONLY way to determine correct pressure. Set the pressure cold and do the tape test after a run down the highway when the road and tire is at nominal operating temperature.
Thing is everyone's stated pressures may be correct.... for the tires and part of the world they live in. For the same tire/vehicle combination, people that live in cold climates will run higher cold pressures than people in hot parts of the world.
Finally, for those that race, how do you determine what's the correct pressure for a race car? Temperature and wear patterns.... no different for street cars, just need more miles.
Yes, I am a tire nerd...
Race Director
Joined: Aug 2003
Posts: 13,421
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From: Lake Mary Florida
To the OP, the 51 psi marking is the maximum. When the tire rolls it causes friction and thus heat, as the air inside is heated the pressure increases. You can pretty much count on a 4 psi increase as a tire operates at temperature.
So if you put 51 in the tire, within a few miles you would have exceeded that as the pressure would rise as the tire heats up.
I suppose on a super hot summer day in the desert you might get a little more than +4 but that's an average.
The best rule of thumb is to adhere to the placard on the driver's door for tire pressure. Note the size if you are not running stock size tires use the stock pressure and monitor the tire for wear (the tape idea is a good one). ±2 psi of 30 is safe to obtain the best tire wear and safety. Remember it will be around 34 for stock sizes while operating at highway speeds.
The last thing you want is a sidewall blowout at any speed as the resulting damage will be more than a tire, likely the wheel and internal/external fender structure.
So if you put 51 in the tire, within a few miles you would have exceeded that as the pressure would rise as the tire heats up.
I suppose on a super hot summer day in the desert you might get a little more than +4 but that's an average.
The best rule of thumb is to adhere to the placard on the driver's door for tire pressure. Note the size if you are not running stock size tires use the stock pressure and monitor the tire for wear (the tape idea is a good one). ±2 psi of 30 is safe to obtain the best tire wear and safety. Remember it will be around 34 for stock sizes while operating at highway speeds.
The last thing you want is a sidewall blowout at any speed as the resulting damage will be more than a tire, likely the wheel and internal/external fender structure.