Had a low tire light on in the SRT the other day, checked all the tires to see if there was a low one and they all read 32lbs. I thought that was about right but checked the door tag to be sure it said 36lbs front and rear, set all tires to 36lbs and light out. Decided to check the limited while I was at it and they all read 36. Checked the door tag on it and it called for 32lbs all around. Lowered them all to 32. My question is why the difference in recommendations between the limited and the SRT, they both weigh about the same and have the same size tires. The ride is definitely better at 32 in both cars but 32 seems to trigger the light in the SRT. I suppose I could just wait for the sensor batteries to go dead and cut the wire, and run whatever pressure I like! Your thoughts?


Jeff K

 

Not about weight so much but probable intended speed. Check with the tire Mfr., they usually say increase pressure with sustained high speed driving. No proof only conjecture on my part.

 

Tire pressures are tuned to the suspension set up is what I would guess. The suspension on the SRT is punishing to some and I guess what's a little more pain to those that can take it. LOL
I added new shocks to mine as I am not a masochist.

 

It's funny my crossfire seems to ride better when back tires are at 36 to 38 & keep the fronts at 32 . That's my liking !

 

On my Limited, door tag is 32 front and 33 rear.

 

On my NA I liked the feel a pound or two more than the door sticker.

 

The door tag (called a Tire Placard in regulations) tire pressure recommendations are the result of multiple factors, but final values come primarily from subjective judgements by vehicle development engineers.

The difference between N/A and SRT pressure values goes with the balance mentioned by others in this thread, that a bit less jarring ride was thought to be what most N/A drivers would be happy with compared to what SRT drivers would want (or put up with). Higher pressures provide handling improvements, fuel economy improvements, and at sustained higher speeds (say, over about 100 mph) or near max loading are important for tire durability / safety.

Requirements for when the TPMS light comes on are called out (for the U.S.) in FMVSS 138 at 25% below the placard recommended pressure. Vehicle manufacturers need to be sure to meet this regulatory requirement, so the system will have a safety factor and may be designed to come on at, say, 22% below recommended pressure.

That's what I can offer about this. Any questions?

Clint

 

Understand, those numbers on the door are for OE tires,go by the tire mfg recommendations. On my Hankooks the max recommended is 50 psi.

 

Welcome to Hankook Tire

 

If you run those tires at that pressure you are asking for wear down the center of the tire, and an unnecessarily hard ride. That pressure would be OK on vehicles that recommended that pressure or some with recommended pressures slightly lower.

 

That's MAX recommended for the tire, not the recommended to run.


Even if you are running non OEM tires, you should always start with the mfg recommended pressure and adjust from there based on tire wear.

 

Do not run with the MAX tire pressure as the tire will expand and can exceed the max pressure and can blow, when I had my Pantera I always had nitrogen filled so very little expansion when driving over 150 MPH got to be safe.

 

Nitrogen expands at the same rate as oxygen, air is 78% nitrogen, 20% oxygen, 1% inert gases (mainly argon and 1% water vapor give or take. So at 150 mph the expansion was the same as if you had used air from a good dry source.

Using nitrogen in your tires is OK, the advantage with nitrogen is that it is contains no water if it comes from the usual cylinder.
Any other claims by its proponents do not hold water or should that be air.

 

Yep, as pointed out by onehundred80, the Ideal Gas Law is pretty good guidance for what nearly all gases do at temperatures and pressures we (and our tires) will experience.

Only different suggestions I offer (as a sometime science teacher, some times auto engineer) is that filling tires with Nitrogen was proposed and serves primarily two beneficial functions:

1) Reduces the amount of pressurized Oxygen inside tires, so reduces the rate and amount of oxidation (aging) of tire rubber, and corrosion of wheels, valves, etc., inside tires.

2) Nitrogen supplied from pressurized cylinders is much drier than normal shop compressed air otherwise used to fill tires, also contributing to reduced corrosion AND decreased pressure variation with temperature.

Wait a minute, what did you say about that Ideal Gas Law and and pressure variation? That Law applies to water VAPOR - but not as water changes state from liquid to vapor. Which happens all the time in the livable temperatures of our Earth home, and happens inside your tires, too. A few drops of liquid water (introduced with that shop compressed air) turns into a much larger volume (hence higher pressure) of gaseous water vapor once you get those tires heated up on the highway.

People selling Nitrogen fills (tire shops) also like to talk about the Nitrogen molecules being so much larger than Oxygen molecules that permeation (leaking through the rubber) is much lower. Little to no evidence exists for that with tires. One study watched 31 different tire models at 30 psi for a year and found Nitrogen fills went down on average 2.2 psi compared with 3.5 psi lost with air. Not so much.

The lesson for today.

Clint

 

I did say a good dry source of air, 1% vapor is not too bad. As to the oxygen, if we consider tires as having a life span of about 6-7 years due to wear and ozone etc. doing its thing on the outside of the tire we can accept less damage being done inside the tire.
I love the nitrogen sellers glib ads, they might as well be selling snake oil.
Never store your tires in a confined space with an electric motor that uses brushes, that will definitely damage your tires, ozone is being generated every time the motor runs and sparks.

 

If the sidewall is stamped with the max cold inflation pressure the Mfr. has already accounted for the tire getting hot.

 

I went out & looked at the sidewalk next to my car & it was stamped 2004. I don't know where I am going to get that much air pressure for my tires.

Just messing with you.

 

SCUBA tank .......

 

Lots of interesting dialogue and knowledge sharing in this thread. For me, I typically always run the tires on my 2007 NA Coupe at 35-36 psig. Seems to give me the best combination of wear & handling. And the low tire pressure light has never come on. Quite a few XFire owners use this tire pressure whether they have an NA or SRT version.

I've got the original OEM Conti's still mounted with no visible signs of any cracking or unusual wear. Rode on them all the way from SC to Eureka Springs, AR & back for this year's CAA6 event. But I'm probably going to change these out in the spring to a new set of Sumitomo HTR A/S P02. Primarily an age issue rather than wear.

Later,

 

All this dialogue has me wondering. Do the Tire Manufacturers reference the maximum air pressure as 50 psig or 50 psia? I'm betting they aren't referencing psia (absolute) which is 14.7 greater than the psig (gauge) figure. Probably related to the general public not being knowledgeable of the difference.

Might actually be an issue for me related to an over-engineering viewpoint.

Later,