Tire Pressure Question

[snippey101]

ok so last night i was chatting with some friends and we got on the subject of tire pressure ended up checking my tire pressure and the front tires were at 25psi rear at 23 psi i have the stock grand touring 18" wheels i looked at the manual and it said pressure should be 35psi but my friends looked at the tires saw the max of 50 psi on the tires and suggested i should run 45 psi i put 40 psi in the front and 37 in the rear

my question is what psi should i run from those who know tires or those who have the tires?

i only have 8500 miles on my car and they are already starting to show some wear on the outside edges of the tires from being under inflated. my gripe is i just took the car to the dealership and paid 60 dollars for them to do the 7500 mile servicing which i though should include changing oil rotating tires and checking air pressure etc.

[Ericzoom]

The correct pressure should be printed on a sticker on the inside of the door jam, and checked when the tires are cold. Never go by what's printed on the tire.

[Jax6S]

Check out the sticky for correct pressure

[posttosh]

ok so last night i was chatting with some friends and we got on the subject of tire pressure ended up checking my tire pressure and the front tires were at 25psi rear at 23 psi i have the stock grand touring 18" wheels i looked at the manual and it said pressure should be 35psi but my friends looked at the tires saw the max of 50 psi on the tires and suggested i should run 45 psi i put 40 psi in the front and 37 in the rear

my question is what psi should i run from those who know tires or those who have the tires?

i only have 8500 miles on my car and they are already starting to show some wear on the outside edges of the tires from being under inflated. my gripe is i just took the car to the dealership and paid 60 dollars for them to do the 7500 mile servicing which i though should include changing oil rotating tires and checking air pressure etc.[/b]

The starting point for tire inflation is to conform to the numbers on the placard or sticker inside the driver's door. You may (probably will) end up someplace else, but start there. Our '6S doen't have the 18" wheels, so I cannot tell you what your placard says, but chances are, it recommends 32 psi, unless you have a MazdaSpeed6, in which case it is probably higher..

For spirited driving, many of us regard the Mazda door sticker recommendation as the today get out your credit card and order one of these and one of these. It will be the best $21 (total) you have ever spent. The pencil gauges at your local gas station are really imprecise and, in addition, are likely to be inaccurate (which is not the same thing). And if you can very easily add air to the tires in the convenience of your own garage or apartment building's parking area, you are more likely to keep your tires inflated to the correct pressure than if you have to seek out the increasingly rare gas station that has public access air pumps, and to do your thing at the very beginning of the day before the tires have heated up to give inaccurate readings.

[Tire Guy]

Our '6S doen't have the 18" wheels, so I cannot tell you what your placard says, but chances are, it recommends 32 psi, unless you have a MazdaSpeed6, in which case it is probably higher..[/b]

So why guess if you don't know? An unusual gaffe for some who seems so well-researched.

The placard for 18 inch-equipped 6s requires 35 psi, not 32. This was confirmed at the dealership two years ago by the simple act of opening the driver's door on a new one, when the first 18 inch-equipped 6s came out. The Mazdaspeed 6 requires 38.

To the OP, given the rate of air seepage (loss of 1 psi per month) and the fact that the weather is beginning to cool (tire pressure goes down 1 psi for every 10 degree drop in ambient temperature), it wouldn't hurt to put at least 37 psi for now. Then, check again in a few weeks. In fact, get in the habit of checking every 3-4 weeks and you won't have the situation where the damage is already done to your tires.

Even if the dealership checks the air pressure in your tires, they will likely get it wrong, because your tires are warm when you drive in to the service lane, and will give a false reading. Lastly, consider putting nitrogen in your tires, which seeps out at a much slower rate (about 1 psi every 6 months) and does not vary much, if at all, in pressure from cold to hot.

[posttosh]

So why guess if you don't know? An unusual gaffe for some who seems so well-researched.[/b]

I was very explicit in stating that I did not know, and I told the OP where to look (the door sticker/placard) to find the correct answer.

The placard for 18 inch-equipped 6s requires 35 psi, not 32. This was confirmed at the dealership two years ago by the simple act of opening the driver's door on a new one, when the first 18 inch-equipped 6s came out. The Mazdaspeed 6 requires 38.[/b]

And yet in your sticky, you give only the 38 psi that is recommended for the MazdaSpeed6, and not the 35 psi that you now tell us is the correct pressure for the 6S. An owber of a 6S who follows --the advice of - Jax6S - to seek out that sticky for the correct inflation pressure for a tire mounted on an 18" wheel could be misled, though it would hardly be the end of the world if he or she inflated the tires to 38 psi instead of 35 psi. Nevertheless, perhaps two years after you discovered the 35 psi data, the time has come to amend the sticky.

Lastly, consider putting nitrogen in your tires, which seeps out at a much slower rate (about 1 psi every 6 months) and does not vary much, if at all, in pressure from cold to hot.[/b]

Ambient air is nearly four-fifths nitrogen, so its gas constant is pretty close to the gas constant for nitrogen. Indeed, the -calculator here- gives the gas constant for air as 0.287 and for nitrogen as 0.297, about a three percent difference. Plugged into the General Gas Law, those gas constants would make the change of pressure as a function of temperature very similar whether the tire was inflated with air or inflated with nitrogen, as you will find if you do the math here (from the Texas A&M website).

[Jax6S]

[Tire Guy]

And yet in your sticky, you give only the 38 psi that is recommended for the MazdaSpeed6, and not the 35 psi that you now tell us is the correct pressure for the 6S. An owber of a 6S who follows --the advice of - Jax6S - to seek out that sticky for the correct inflation pressure for a tire mounted on an 18" wheel could be misled, though it would hardly be the end of the world if he or she inflated the tires to 38 psi instead of 35 psi. Nevertheless, perhaps two years after you discovered the 35 psi data, the time has come to amend the sticky.[/b]

Why don't you actually READ the sticky? You missed something.

As far as your nitrogen theory goes, I don't really care about the math. I know what the actual results of running nitrogen in my tires are. Go away.

[posttosh]

Why don't you actually READ the sticky? You missed something.[/b]

Yes. I missed where you stated that 35 psi is ever recommended for tires mounted on Mazda6S vehicles with 18" wheels. Unless you have modified the post this afternoon, the only pressure listed there for any 18" wheel is 38 psi. The 35 psi you have revealed to be the recommended pressure for Mazda6Ss equipped with 18" wheels is completely missing from your listing.

As far as your nitrogen theory goes, I don't really care about the math. I know what the actual results of running nitrogen in my tires are. Go away.[/b]

Theory? The laws of physics govern the pressure and volume of gases as a function of temperature. The parameters of those physical laws are available to us because of the discoveries of Edme Mariotte and Robert Boyle (Boyle's Law, 17th century) and of Joseph Louis Gay-Lussac and Jacques Charles, building on the work of Guillaume Amontons, in the 18th century (Charles's Law). Boyle's Law and Charles's Law were combined into the General Gas Law, sometimes called the ideal gas law, which is actively taught in colleges and universities today. I encourage you to use Google (or your favorite alternative search engine, or your local public library) to discover more.

The laws of physics are not political; you cannot change them by bluster or by calling sycophants to rally to your cause. The laws of physics will not "go away" no matter how loudly you demand that they do. Cf. Canute and the tides. Your assertions about the differences between the behavior of nitrogen and the behavior of atmospheric air (which is about four-fifths nitrogen) with changes of temperature appear to contravene the laws of physics, ind therefore must be regarded as largely fanciful, comparable to Roberto Madrazo's remarkable feat of running nine miles in 21 minutes in this past week's Berlin Marathon.

[Tire Guy]

Yes. I missed where you stated that 35 psi is ever recommended for tires mounted on Mazda6S vehicles with 18" wheels. Unless you have modified the post this afternoon, the only pressure listed there for any 18" wheel is 38 psi. The 35 psi you have revealed to be the recommended pressure for Mazda6Ss equipped with 18" wheels is completely missing from your listing.[/b]

WRONG, DIPSHIT! I haven't modified that sticky in a long time and here is the information right in front of your eyes that you missed:

18" (original equipment wheel is 18x7)

215/45R18 7.0"-8.0" 35 psi (6s, 6i), 38 psi (Mazdaspeed6) *original equipment size*
225/40R18 7.5"-9.0" 38 psi
225/45R18 7.0"-8.5" 38 psi
235/40R18 8.0"-9.5" 38 psi
245/40R18 8.0"-9.5" 38 psi

Why don't you fucking admit when you're wrong, asshole!