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Old 02-18-2012, 09:44 AM   #15
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Unbeknownst to most is the correct formula used to determine the recommended cold tire air pressures of any passenger automobile. Its pretty simple

The gross vehicle weight divided by one hundred (GVW in lb/100) equals the recommended tire PSI when cold.

For example: Your Vehicles weight with a full tank of fuel plus the load capacity of passengers and cargo. Lets say yours is 3700 pounds.

3700/100= the recommended cold PSI is 37.

The cold pressure recommendation is generally within 5 pounds on the tires PSI once it has warmed to road surface use tempature's. Heavier use such as road racing and drifting may cause the tire PSI to increase above these of course. Over loading the weight carrying capacity of the vehicle will also cause the tires PSI to increase and may cause failure,

Obviously there are lighter and heavier Gross weights among automobiles using the same size tires and thus the variances noted of a allowable maximum tire pressure on the tires sidewalls.

Its also important to remember that proper alignment settings of the vehicle is crucial to even tire wear and safety. It plays a role in how the tire makes contact with the road surface and thus the amount of heat created inside the tire, raising the PSI. Theres is a significant connection between tire inflation PSI and improper caster and toe in settings. Improper settings increase the amount of heat/friction created that directly affects the tires by inordinately increasing tire PSI under use, in as much safety and longevity.
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Old 02-18-2012, 10:18 AM   #16
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open owners manual...if that does not work..open drivers door..there is a sticker with info. 36psi
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Old 02-18-2012, 10:22 AM   #17
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See attached max pressure. 50 PSI it is. Think I found my own answer below. Looks like the setting should be at 36 cold. Will make the adjustment. Thanks for weighing in. I love this site.

Maximum Inflation Pressure
A tire's maximum inflation pressure is the highest "cold" inflation pressure that the tire is designed to contain. However the tire's maximum inflation pressure should only be used when called for on the vehicle's tire placard or in the vehicle's owners manual. It is also important to remember that the vehicle's recommended tire inflation pressure is always to be measured and set when the tire is "cold." Cold conditions are defined as early in the morning before the day's ambient temperature, sun's radiant heat or the heat generated while driving have caused the tire pressure to temporarily increase.
For the reasons indicated above, It is also normal to experience "hot" tire pressures that are up to 5 to 6 psi above the tire's recommended "cold" pressure during the day if the vehicle is parked in the sun or has been extensively driven. Therefore, if the vehicle's recommended "cold" inflation pressures correspond with the tire's maximum inflation pressure, it will often appear that too much tire pressure is present. However, this extra "hot" tire pressure is temporary and should NOT be bled off to return the tire pressure to within the maximum inflation pressure value branded on the tire. If the "cold" tire pressure was correctly set initially, the temporary "hot" tire pressure will have returned to the tire's maximum inflation pressure when next measured in "cold" conditions.
A tire's "maximum inflation pressure" may be different that the assigned tire pressure used to rate the tire's "maximum load." For example, while a P-metric sized standard load tire's maximum load is rated at 35 psi, many P-metric sized standard load performance and touring tires are designed to contain up to 44 psi (and are branded on their sidewalls accordingly). This additional range of inflation pressure (in this case, between 36 and 44 psi) has been provided to accommodate any unique handling, high speed and/or rolling resistance requirements determined by the tire and vehicle manufacturers. These unique tire pressures will be identified on the vehicle placard in the vehicle's owner's manual.
The tire's maximum inflation pressure is indicated in relatively small-sized print branded near the tire's bead (adjacent to the wheel) indicating the appropriate value. Because tires are global products, their maximum inflation pressure is branded on the tire in kilopascals (kPa) and pounds per square inch (psi). These values can also be found in the industry's tire load & inflation charts.

I did the math, not sure if that's allowed in this Forum, but anyway...our cars weight a little over 2,000 lbs so each tire is carrying 500lbs, well under the carrying capacity of the tire. Even adding in passengers, cargo and throughing the beastie into hard corners, those tires don't get near thier max pressures. Nail in the coffin shows 36 is a good number.
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Old 02-18-2012, 10:37 AM   #18
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I believe the pressure listed in the book and door pillar is for a maximum weight condition. Most people people not use there vehicle like that. If the tires are inflated to allow for maximum weight, and you only carry yourself 95% of the time, the tires will wear out prematurely, and you will not have maximum traction. Slightly vary your pressure to match your load.

This is much more important on a pickup for instance.....70 or 85 psi is not needed to carry myself. Now if you drop in 3500lbs in the box, you better air them up.....which I do.

If extra tire wear is not an issue with you, then air them to the manufacturers sticker all the time. Just my 2 cents.
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Old 02-18-2012, 11:19 AM   #19
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Quote:
Originally Posted by One-Bad-SS View Post
I searched but have not found the answer to this question....

The Pirelli P-Zero's sidewall states a max pressure of 50 PSI, the car's tag says 36 PSI, it appears the TPMS goes off at under 32 PSI...I was always taught not to follow the tag cuz you never know if the tires have been changed from manufacture's specs...so I've always adheared to the sidewall. Is everyone else doing so? Not looking to wear these out in 15K.

It would seem GM set the low pressure warning for these tires too low, if that is the case. Cold, they should hold about 45 PSI and will warm up close to max. Logical right? Can we set the TPMS to alert for higher pressures, say 40PSI closer to the tires actually rating?

OK, let's hear it. Thanks.
Whoever told you not to follow the Manufacurer's specs was mistaken. You should always follow the recomendations on the door jamb sticker or owners manual. The max pressure on the sidewall of the tire is just that, the max pressure that you can inflate the tire, before it becomes an explosion hazard.
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Old 02-18-2012, 11:30 AM   #20
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If you have the tires filled with nitrogen you will only see 2-4psi difference between the coldest days and the hottest. They also lose less pressure over time.
Actually this is false. Pressure change is a set amount across all gases. It doesn't change from one gas to another. Nitrogen, oxygen, helium, hydrogen, etc they all expand and contract with temperature at the same rate.

Also air is over 70% nitrogen

Quote:
Originally Posted by suplex View Post
I had always thought it was 1 PSI for every 10 degrees temperature difference. However, if that's true, what do they consider the "starting" temperature?

Let's say they refer to 70 degrees as the initial temperature, and your car is supposed to be at 35 PSI when it's 70. I would assume that means that if it's 40 degrees out, you should set your tires to 38 PSI, and if it were 100 degrees out, then set the tires to 32 PSI. So if it's hotter out the PSI goes down, and if it's colder out the PSI goes up.

Am I correct according to my above example?
The friction of driving will heat up the temperature of the tires to the same temp eventually. The only difference is the length of time it takes to get there. During the winter you want to have your tires a little less PSI then summer because during the winter there will be a larger temperature change.
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Old 02-18-2012, 11:38 AM   #21
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Originally Posted by One-Bad-SS View Post
I did the math, not sure if that's allowed in this Forum, but anyway...our cars weight a little over 2,000 lbs so each tire is carrying 500lbs, well under the carrying capacity of the tire. .

Ummm, you may want to double that figure, the camaro weighs in at close to 3800 lbs.
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Old 02-18-2012, 11:41 AM   #22
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Ummm, you may want to double that figure, the camaro weighs in at close to 3800 lbs.
With a full tank of gas and a driver it is almost 4,200.
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Old 02-18-2012, 11:45 AM   #23
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I go with the 36 cold off my own tire gauge...the dash reading has never matched yet!
^^This^^
My dash reading always read 1 or 2 PSI high.
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Old 02-18-2012, 11:49 AM   #24
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i keep mine at 33 cold.
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Old 02-18-2012, 12:46 PM   #25
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Summer tires = 36 psi

Winter tires = 38 psi

Nitrogen is used because it does not bleed off as fast as regular air.
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Old 02-18-2012, 01:34 PM   #26
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Originally Posted by Caspers2SSRS View Post
Unbeknownst to most is the correct formula used to determine the recommended cold tire air pressures of any passenger automobile. Its pretty simple

The gross vehicle weight divided by one hundred (GVW in lb/100) equals the recommended tire PSI when cold.

For example: Your Vehicles weight with a full tank of fuel plus the load capacity of passengers and cargo. Lets say yours is 3700 pounds.

3700/100= the recommended cold PSI is 37.
dont do that. just chance that works on passenger cars, but not so much on a 80,000 lb semi. or maybe a 10,000 lb monster truck (those run 4 psi). see the problem with that? the psi is determined by several things, not just load. every tire that that can fit on these cars will work just fine at 36 psi. 35 is generic for all tire guys.

nitrogen is used mostly because of the reduced expansion, but also because of the larger sized molecules to reduce air escape.

and comrando
Boiling point????? what are you doing to your tires???
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Old 02-18-2012, 01:37 PM   #27
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Sorry about the math thing, see my build sheet attached. Apparently they gave the metric weight. I just checked the car's data plate and it's about 4600 LBS. Still 1,150 per tire is well under.

I knew we shouldn't talk math.
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Old 02-18-2012, 02:22 PM   #28
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Sorry about the math thing, see my build sheet attached. Apparently they gave the metric weight. I just checked the car's data plate and it's about 4600 LBS. Still 1,150 per tire is well under.

I knew we shouldn't talk math.
Yea. Our cars are built in Canada - so metric. That is one thing I love about our cars though, bolts and nuts are all in mm. SO much easier to work with.
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