We just picked up our GS this week and the OnStar report stated that our tires are over inflated at 35 psi vs. factory spec of 30. They came from the dealer with nitrogen instead of air. Is there a reason or benefit from over inflating with nitrogen, or was it simply a dealer error.

 

No, they're simply overinflated.

 

dealers usually don't care to get it perfect. 30psi is correct

 

Tire jockeys never look at the label for psi. Usually overinflated. I always check tires on a new car when I get it home.

 

There was a point at which 35 psi. was the recommended cold fill for a C7. I don't know if it has changed back to 30 psi cold or not. What does the label on the door frame say it should be?

That, and the Onstar report could have been reading your car after the tires were warm, which very well could account for the 35 psi, which is about right if warm.

 

30 psi on b pillar

 

The next question is what does a hand-held gauge say when cold? Make them 30, and call it a day.

 

In '14, the recommendation started with 30 psi, and in a subsequent model year went up to 35, likely for gas mileage ratings. I don't know if they've dropped it back down or not by '17.

 

Yes

 

I wouldn't use nitrogen in my tires. I wonder why they did. It would be good to know if you can find out. Thanks.

 

Thanks for the feedback guys. As stated above, the pillar says 30 psi. I can see that using 35 might increase mileage, but I wonder whether there's a negative effect on traction or tire wear. I'll see if the owner's manual mentions anything about using nitrogen.

 

Filling with nitrogen (and charging for it) is a scam. The air we breathe is largely nitrogen already. The only good thing with a nitrogen fill is that (usually) it's filtered for less water vapor.

 

The base car with 18/19 inch wheels calls for 35 psi cold. (I will guarantee that that will wear out the center of the tires prematurely.)

Z51 19/20 inch wheel calls for 30 psi cold.

Also, nitrogen is a scam.

 

Label on my '17 Grand Sport says 30.

 

I talked to a guy yesterday about nitrogen in tires and he didn't like the idea of driving around with an explosive gas in his tires.

They walk among us!

I understand nitrogen in aircraft tires where the radical temperatures deltas allow more moisture to precipitate and in race cars for better pressure control, but the 78% ambient content is fine with me.

 

This. Dealers and tire shops push it because it's hugely profitable. It provides no real advantage in passenger car tires.

 

Tirepressure advice is to give the tire a deflection at wich no overheating when realy driving the speed its calculated for.
If any spot of tire goes above a sertain temperature, to many sulfur bridges form and the rubber gets hard, and crackes in next bendings of it by the deflections. Once the damage is done to the structure of rubber, there is no way back, and bending of rubber will slowly ( or quick) tear the tire apart, and suddenly blowing tire, with the missery that goes with it..
Realise that every segment of the tire deflects and flexes back about 10 times a second when driving 50m/80km/h.
this produces heat.
The more cycles a second , the more heatproduction a second. Cooling down of rubber is dependent on temperature differences between>>
Rubber and surounding rubber.
Rubber and in and outside tire air.
Inside and outside tire air.
Not on speed because on the ground no speed and at the top of tire more then 2ce the speed of the axle and car, but in the wheelcase where practically no airstream.

The deflection is made by the combination of weight on tire and the pressure in the tire, and I am able to calculate a save advice .
You only need to give me 100% acurate data for it( so never possible).

The 35 psi is the At-pressure of a standard load P-tire.
AT-pressure I call it because LT tires give it like that on sidewall.
On P-tires the maximum alllowed cold pressure is given on sidewall, of betweeen 44 and 51 psi for standard load and for XL/reinforced/Extraload sometimes up to 60 psi.
The AT-pressure is the pressure for wich the maximum load of tire is calculated for up to 160km/99m/h. The allowed pressure above that 35 psi is used to highen up the AT-pressure with a system depending on speedcode of tire.

Nowadays the car-makers advice higher pressure , behind mostly that AT-pressure or even maximum allowed cold pressure of tire. It is all for responcibility reasons, today a car- or tire-maker can be held responcible , when a tire blows, even if its because someone overloaded extremely. To cover this they advice this high pressure.
But this does not mean, that to laws of nature, you can not use a lower pressure.
Especially when only driver and a little load in car, the pressure can be lower savely, and mostly when motor in front , front higher pressure then rear. This normal use advice was prettty common in Europe before 2000 ( before the Ford-Firestone Affaire) and normal advice was calculated for axleloads determined by the car-maker for 3 persons and a little load in the car, and as I already wrote , mostly front higher the rear.
Fully loaded the GAWR's where used to fill in the formula.
Nowadays also in Europe the normal use is sayd to be for 4 persons and load , and for that they fill in the GAWR's ( so what before 2000 was the fully loaded advice).

About Nitrogen filling ( wich is not explosive), I made a document and will give copy of it here.

Filling car tires with normal air is better then with Nitrogen.

A. Oxygen, which makes 21% of normal outside air, in volume absorbs more energy to rise a degree in temperature then Nitrogen.
Difference is only 1% , but in selling argument is sometimes mentioned in energy/ mass so Joule/kg.degr. Kelvin and there Nitrogen wins by 13 % . Tires though are filled with a certain volume so that counts .
This selling argument can be de-myth simply by the fact that its untrue.

B. Water in a tire transports the heat more then dry air or Nitrogen-filling ( which is by its production process free of water).
Not for no reason water is used in central heating and cooling system of cars as main substance.
The tire inside will have lower temperature which is better for cooling down the important spots of the tire wich have to keep a low enough temperature to prevent hardening and damage in next bending of those parts.
When incidentally the tire inside gets hotter ( sunshine on tire or severe braking, or high ambient temperature) more water goes over to gas wich highens the pressure inside the tire more then dry gas. Also then more water as gas in tire so even better cooling.
So advantage of water is 2 ways when needed, better cooling and lesser heat production.
Disadvantages of water as oxidation only happen at outside . Tire specialists will confirm that when they remove a tire of the rim , the rim inside the tire is not corroded.

C. When a Truck, transporting flammable substance, is on fire, the little oxygen in a tire will , when exploding ,probably even kill the fire .
In tunnels experiments where done to kill the fire with air current , and it worked, despite the oxygen in it.

D. A normal car tire up to truck tire is not a race-car tire, for which a constant as high as possible contact area, so best grip , is needed to give half a second better round time.
For that reason they are filled with as dry as possible air or Nitrogen.
That this gives low lifetime is not important, and for a normal car tire it is.
Often blowing tires at racing, is seen there as collateral damage, but we don’t want that for normal car tires.

E. A car tire is also not a airplane- tire in which the water can freeze and when landing this can give misbalance , which can lead to tire-failure or accident, when suddenly going from zero to about 200m/h.
The water in a car tire does not freeze that often, and when it does and the car begins do drive , it begins slow and the misbalance is not a big problem . then pretty soon, certainly when on speed the ice is melted to water and problem will be gone.

F. When filled with normal air the user is aware of checking the tires regularly, which takes better care of a saver tire, because regularly filled up to the right pressure and optical checking of the tire .
The illusion that Nitrogen filling makes the tire to loose almost no pressure, which is often exaggerated to 5 times , while in real a poor 2 times and then only in the very beginning, makes the user less caring .
The tire also looses air when hitting pavement so temporary leak between edge of tire and rim, Filling with whatever gas-combination won’t prevent that.
This idea is even stronger when TMPS is used which is often inaccurate , so the user thinks to maintain the right pressure, while really riding with to low pressure for longer time, which can lead to tire damage. Aftermarket sensors for TMPS when screwed on the valve can give leakage trough hole where valve is placed , because of the bending of the valve by centrifugal forces at speed.

G. Filling with normal air is always possible, even with a bicycle- pump, is only to fill up so won’t take that long. When you keep driving with to low pressure , in order to find a place to fill with Nitrogen, you damage your tires. So just fill up with normal outside air, even when the tire is filled with Nitrogen. Then also you don’t need to have the tire refilled with pure Nitrogen by your tire specialist when at home again, not worth the trouble, and normal air is even better then nitrogen because of the water in it.

Greatings from a "Dutch Pigheaded Selfdeclared Tirepressure-Specialist"
Peter

 

Nitrogen is an inert gas.

 

FWIW. Worked for the leading industrial gas producer. Our highest selling product was oxygen, which we sold to steel mills directly from large onsite gas liquefaction plants delivered in a pipeline. We had to liquify air and were left with 78% liquid nitrogen, which we could not sell in the volumes produced. After all that work and wasted energy cost, much was just vented!

Our highest profit margin product was argon, a byproduct! At the time we had 50% of the North American production capacity of that gas (it's only 0.9% of the air.) As Director of Market Development for the welding business, that was my groups main goal, help keep argon demand higher than supply! Our gas market share was 30 to 33% but the remainder ~20% volume we produced was sold wholesale to the other industrial gas producers. (Perhaps of interest it is now in short supply and has tripled in price in the last few years!)

My counterpart was developing info to have nitrogen used in car tires and to replace air compressors in large industrial plants. Although there is a benifit for some applications, like large truck fleets, for car tires not much, except to make the dealer additional profit!

For large truck fleets that recap their tire where the body can last serval hundred thousand miles, the reduced oxidation has a benifit. Airplane tires are an interesting application. They fill the gas shocks with ~2500 psi as I recall. We sold special ~5000 psi large nitrogen cylinders for that application. When they got below 2500 psi they were useless for shocks, so why not fill the tires and use the lower pressure nitrogen to fill the tires. It is dry but so would air be if put through a dryer!

Bottom Line: Follow the Money!

 

jadatis... thanks for taking the time to supply our membership with the details you provided. I think that I will deflate my tires to the 30 psi recommended for our GS, then follow your advice (G) to fill them with air as needed in the future.