Walt White Coupe

For a couple years (2016 being one), GM recommended 35 psi on the "standard" 18/19 inch wheels in an effort to get better fuel mileage.

Vipula AD

You are right, it is 10 degrees. Thank you for the correction.

mbaessler

My 2017 says 35psi on the door sills. I set them at 30. Getting great wear.

Power Hawk

My 2016 Stingray door jam states 30 PSI cold. So does the owner's manual. I adjust them a couple times a year depending on the temp. outside.

Walt White Coupe

^^^
You don't have the standard 18/19 inch wheels, you have 19/20 inchers.

Apocolips

My 2014 Stingray door sill said 30PSI and my 2017 Grand Sport says 30PSI, my friend's 2018 Z06 also says 30PSI. Are you sure yours says 35PSI?

Walt White Coupe

^^^^
It depends on what size wheels came with the car. 18/19's were 35 and 19/20's have always been 30.

wayback

My 2016 with 18/19 wheels says 35 psi on the door. TPMS alarms at 28 PSI . It would be going off 2 times a week here in Texas if I ran at 30 PSI.

I usually run at 32 PSI to avoid the TPMS warning.

Kingtal0n

Weight is an influence- more weight, use more pressure to maintain tire shape. Instantaneous stress such as bumps is also a factor as it rapidly deforms tire.
Another consideration is air leaks out over time, little bit. 30psi today is 29.98psi tomorrow or whatever.
Temp of road and friction is another factor. hot weather + friction is more temperature than just hot weather sitting still in a driveway, pressure will be higher after driving in the same weather condition as when static.

All this taken together means I always run an extra couple few psi. For example instead of 30 I will use 32psi or 33psi. This helps with gradual bleeding down, instantaneous stress, additional weight, whatever

owc6

I personally saw a 12 psi gain on one day.

I started out from Kansas City, pre-dawn cold rain at 30 psi, Drove west and out of the rain as the temp rose, ended up in Denver mid afternoon and the temp was pushing 100. The psi was 42.

Needless to say, I let some air out of the tires.

These low profile tires can produce a lot of pressure change from temp (and altitude). I've seen noticeable difference from one side to the other on a sunny day just from the sunlight hitting only one side of the car.

owc6

It was strictly a paper-work change to improve milage stats.

Nothing physical changed about the car or the tires, or magically changed back after.

jadatis

You should not have let some air out, gives more deflection, so heatproduction, so higher temperature of the gascompound in the tire, so higher temp of the tire-material.
And that is what counts, tire-material may not overheat, otherwise little cracks are made, that crack further in time by the mechanical forces, untill mayby after 2 years that far, that tire blows or treath seperates. Then that 2 years ago situation of overheating is long forgotten.

If 33psi cold filled at 70 degrF, my made list gives 11 degrF/ 1psi.
So if 30 psi cold filled at 48 degr F and rising 11 degrF/ psi, temp in tire is 33psi at 70 degr F and 9 psi more is 9x11degrF is 99 degr Hotter is 169 degr F ( aproximately) pretty hot still.
Makes me suspect external factor like sunshine on tire, and or higher speed used.

Here the list, for cold fillled at 70 degrF and degrF per psi change behind it.
Some write 2% psi change per 10 degrF, and is already more acurate.
Here my made acurate enaugh list for the goal, if you use it going from 60 degr F cold, it wont give dramatic differences.

70degrF./degrF/psi
20 psi/ 15,5F/psi
21 psi/ 15F/psi
22 psi/ 14,5F/psi/ EUR lowest official
23 psi/ 14 F/psi
24 psi/ 13,5F/psi
25 psi/ 13,5F/psi
26 psi/ 13 F/psi/ US lowest official
27 psi/ 12,5 F/psi
28 psi/ 12,5 F/psi
29 psi/ 12 F/psi
30 psi/ 12 F/psi
31 psi/ 11,5 F/psi
32 psi/ 11,5 F/psi
33 psi/ 11 F/psi
34 psi/ 11 F/psi
35 psi/ 10,5 F/psi
36 psi/ 10,5 F/psi/ referencepressure SL
37 psi/ 10 F/psi
39 psi/ 10 F/ps
40 psi/ 9,5 F/psi
42 psi/ 9,5 F/psi/ referencepressure XL
43 psi/ 9 F/psi
45 psi/ 9 F/psi
46 psi/ 8,5 F/psi
49 psi/ 8,5 F/psi
50 psi/ 8 F/psi
53 psi/ 8 F/psi
54 psi/ 7,5 F/psi
58 psi/ 7,5 F/psi
59 psi/ 7 F/psi
63 psi/ 7

jadatis

I made a word document to use in RV forum, but also counts for personscars, you only have to translate it to the lower used pressures ( 35 psi instead of example 80 psi).

Then you see that you can let the cold filled pressure swing with temperature, and still no overheated tires.

Here it is, dont pin me down on the exact temperatures.

Why pressure should be calculated back to 70 degrF
That pressure of tires is related to the temperature of gascompound in tire, is fact.
But that for instance you need for 100 degrF ambiënt temperature higher cold filled pressure, then for 70 degrF, and for 40 degrF can do with lower pressure then for 70 degrF, is a conclusion I took, and is the discussion here.
I am going to explain how I came to this conclusion.
I once had a long telephone call with a man from Vredestein , and he explained that goal of pressure determination and maxload of tire, is to not overheat them.
He explained that the molecule-structure of natural rubber can be compared to spagetti, and if you push your nail in it the print remains . Then the “ spagetty-strings" slide over each other.
Tires are vulcanised at the end, and this makes sulfurbridges between the “ spagetti “ wich makes the rubber flexible, so when you push your nail in it , the print directly dissapears, the rubber goes back to its original shape.
The rubber needs a certain amount of sulfurbridges, but to much makes the rubber hard, and if then bending of the rubbber, by the deflection and flexing back every cycle, gives small cracks, wich dont disapear ( logical)
When rubbers temperature goes above a critical temperature, to many sulfur bridges are created,and rubber hardens. Wich temperature this is I dont know, but the vulcanising proces is at 170 degrC is 340 degrF.
Rest is my Einstein way of thinking, and because my average IQ is probly half of that of Mr Einstein, change is 2ce as much that my theory is proven wrong once.
The temperature of the rubber is created by the balance between heating up a second and cooling down a second.
When you begin driving at cold pressure in ambiënt temperature of 70degrF, in-and out-side-tire air and all the rubber of tire is 70 degrF. The heat-transport then is zero. In basics Maxload is calculated for reference-pressure ( E-load 80 psi) and reference-speed ( P- and LT tires mostly 99mph/160kmph) so when you drive that speed constant with maxload on tire and reference-pressure cold filled in tire, no overheating of any part of tire. I practice a bit more complicated.
Yust for the example say you then drive 99mph constant speed, with referencepressure of 80psi ( E-load) and maxload on tire. In the beginning cooling down is marginal, because temperaturedifferences between rubber of tire and in and outside tire gascompound is still zero, but driving long enaugh rubber yust below its critical temperature in the middle of the thickest parts of tire, and at the edges , where rubber meats the gascompound, lets say 300 degrF.
Inside tire gascompound I state as 140degrF. Outside the tire always air 70 degrF

Then temperature difference between edges of rubber and inside tire gascompound 300-140=160 degrF., and between outside tire air 300-70=230 degrF. Also the inside tire gascompound is cooled down mostly trough the rimm to the air outside the tire with only 140-70= 70 degr temperature difference so 160/70 = 2.28 times as much cooling down by the better transport trough the metal rimm.
So coolingfactor then inside tire 160/230=70% of outside tire. In and outside tire Together 230+160=390 degrF
The rubber temperature then stays in balance ,so heating up factor also 390 degrF.
But the deflection , so heatproduction a second at that speed, then is lesser then at cold pressure, because temp in tire 140degrF, wich gives higher pressure. 80 psi filled at 70 degrF becomes 92.9 psi at 140 degrF.

Now situation ambiënt temp 100 degr F.
Temp of rubber still max 300degrF.
Temperaturedifference rubber and outside tire air 300-100= 200 degrF
Temperaturedifference rubber and inside tire gascompound 70% of outside 200= 140 degrF.
Together200+140=340 degrF worth of cooling capacity
So temperature inside the tire 300-140= 160degrF.
So at 70degrF ambiënt temperature 390degrF worth of cooling capacity.
At 100degrF ambiënt temperature 340degrF worth of cooling capacity.
This has to be compensated by lesser heatproduction a second by lesser deflection of tire is 340/390=87.2 % of the heatproduction at 70degrF ambiënt temperature.
Now if 80psi filled at 70degrF
At 100F/85.4psi
At 140F/ 92.5psi
At 160F/ 96.1psi
So 92.5/96.1=96.25% of the surface on the ground then at 70degr F driving 99mph with maxload on tire. Rule of tumb I determined gives 0.9625^2= 92.64 times the deflection . If heatproduction goes lineair with the deflection ( I hope and not more), this gives 92.64% of the heatproduction.
Cooling down is 87.2% , so heating up is even still more then cooling down, if 300degrF is the critical temperature of rubber .
If you then lower the 85.4psi cold pressure at 100degrF ambiënt temperature to 80psi, as is stated to be allowed because cold filled is at ambiënt temperature, the pressure becomes at 160degrF 90.2psi instead of 96.1psi ,wich gives more deflection so heatproduction a second, so rubber of tire temperature goes above 300degrF, and it hardens and cracks.
In fact , because at 100degr heating up factor is 92.64% and cooling down factor is 87.2% at 100degrF ambiënt temperature of the 70degrF situation, you should even pump the tires up a little to give lesser heatproduction a second., so heating up factor goes also to 87.2

Now ambient temp 40degrF.
Outside tire 300- 40= 260 degrF worth of cooling down factor.
Inside tire 300- ( 70% of 260= 182 degrF)= 118 degrF inside tire gascompound
So cooling down factor 260+182= 442 degrF
This is 442/390= 113.3% of coolingdown factor then at 70 degrF.
Then at 40 degrF 80psi filled at 70 degrF becomes
74.6 psi cold and at 118 degrF inside tire temp 88.6 psi
92.5psi/88.6 psi = 1.044 times more surface on ground gives 1.044^2= 1.09x more heatproduction.
So when colder tires rubber stays cooler then 300degrF when driving 99mph with maxload and reference-pressure on tire. So for safety not needed to fill up the cold pressure of 74.6 psi to 80 psi , with difference to hot temps, that you may do so for riding quality and fuell saving.
So I think the lists for filling higher at 70 degrF in a heated garage , to get the 80 psi in this example at for instance 20 degrF is because at those extreme cold ambiënt temperatures the deflection gets to much so for instance snake-bite.
Then its not anymore to prefent overheating, but for riding -quality.

orca1946

jadatis WOW! That's tons of info! Thanx

Walt White Coupe

Talk about overkill. The question was: What is the correct pressure?

And the answer is, whatever it says on the door jamb.

jadatis

​​​​​​
That formula I also use to make my list, but assume volume to stay the same, wich it practically is for a presurised tire.
But you forget to calculate with absolute pressure instead of overpressure, what gauge gives.
In my list I assume 1013 mb is 14.69 psi ambiënt pressure.

And the Fahrenheit version of Kelvin is to Celcius, is the Rankin scale.
Zero Degr Rankin is minus 458 deg F.

But to come back at the opening question.
Needed pressure ( at 70 degrF or whatever it realy is), is dependend on load on tire, and maximum speed used ( and wont go over for even a minute.
Can make pressure / axleload lists for speed used and reserve build in.
Determined that using 90% of loadcapacity for the pressure for 160kmph/99mph. gives max reserve with still acceptable comfort and gripp

Followed the link and also on that site, they calculate with absolute pressure instead of overpressure, as gauge gives. Contacted them about it.

Zahmed

How do you fill your tires accurate to 1-2 PSI? What do you use? The air pumps at gas stations are tough to get right.

Vipula AD

I use this;
with a small air compressor like this I have at home;
The combination is as accurate as the gauge on the car and that is good enough for me.

When I AutoCross, I take this with me;
Works very well and has auto shutoff at set pressure, not as accurate, I set it to 32psi and it shuts off at 30psi on the tire. Has been working well for about a year now.

Kingtal0n

Thats another good point.

I have five thermometers from fishersci and five tire pressure gauges

and all of them read differently

Its impossible- impractically impossible- to set some exact number of PSI. That is also why I shoot just over my target: "be sure"

HOXXOH

I generally set tire pressure to get the best directional footprint for the current traction needs.
30 PSI cold on factory stock tires is fine for average Corvette drivers who are driving in average conditions on average roads.
Once any one of those conditions change, for any reason, ideally the pressure needs to change too.