correct tire pressure??
 

I checked my tires pressure today and in front i had 38psi and 37.5 psi on the back i gues that guys that mounted my tires didnt give shit about correct pressure...car felt crazy especially when changing lanes.....
so my question is what is best tire pressure for 235/40 tire that is on 18x8 rim??
I lowered fron to 35 and rear to 33 ...1psi highger than specs

IMO 35 up front is too much. epecially on a FWD car. i would just run 33 all around b ut maybe others can chime in.

I'd start with what the tire says and then lose a few PSI.

don't tires just say the max psi that the tire will withstand? i could be wrong?

Correct. The rating on the side of the tire is the max rating that the tire can handle. The pressure that the tires should be inflated to is listed on the sticker inside the drivers side door.

Edit: 34psi front 32psi rear (MS3). Also, if you fill your tires with nitrogen instead of air they will maintain pressure better and leak less over time.

but those specs are for stock tires

it is for all the tires you run, thats why when you look at a tire it only says the maximum pressure it can withstand, not what pressure to run them at, because every tire is made for more then just one vehicle. and every vehicle is designed to run a specific pressure, hence the sticker on the drivers door.

ok thats my point i will run 34 front and 32 back or should i have less up front for better traction??

id say stay with the car specs to achieve the best fuel millage

also with "some" tires less pressure don't necessarily mean more traction.

So you're saying regardless of the brand or size tire you're running, your PSI should be set at what the sticker on the door says? That sounds awkward.

Correct. The pressure rating on the door has to do with the gross vehicle weight. remember that when a tire gets up to speed it heats up quite a bit and the pressure changes. The more weight in the car combined with highway speed and over/under inflated tires can cause a rupture. The rating on the side wall of the tire is the max inflate pressure for that tire. if the rating on the side wall is under the suggested inflated pressure on the door then the tire is incorrect for the car.

Also the tread pattern is designed to contact ideally within a certain range. Over/under inflating a tire will cause the tread to contact incorrectly. Over inflating will cause the center of the tread to contact while the outer edge will lift. Under inflating dose the opposite, the center lifts and the pressure is on the outside of the contact patch. You want to use the proper inflation so that contact and pressure are even. Thus giving you the best traction/ MPG(L/110km) possible, as well as tread life.

The exception to the rule would be drag radials.

how does all this change with wider tires and wheels? Let's say I have 245/40/18 on 8" wide wheels..... or would I still use the same Psi?

Where did you get this information? I'm not trying to start shit here but I've never done it this way. If I've been doing it wrong I would like to know but I'm going to need something more than a post from some guy on the internet.

I remember reading it else where but here it is from Wiki.

Inflation pressure

Tires are specified by the vehicle manufacturer with a recommended inflation pressure, which permits safe operation within the specified load rating and vehicle loading. Most tires are stamped with a maximum pressure rating (for USA only). For passenger vehicles and light trucks, the tires should be inflated to what the vehicle manufacturer recommends, which is usually located on a decal just inside the driver's door, or in the vehicle owners handbook. Tires should not be inflated to the pressure on the sidewall; this is the maximum pressure, rather than the recommended pressure.[4]

If tire pressure is too high, the tire contact patch is reduced, which decreases rolling resistance. However, ride comfort is reduced, but traction is not always reduced, stopping distance is not always increased. [5] Also, going above max sidewall pressure rarely results in the center of the tire wearing more than the shoulder.[6]

If tire pressure is too low, the tire contact patch is increased, increasing rolling resistance, tire flexing and friction between the road and tire. This "underinflation" can lead to tire overheating, premature tread wear, and tread separation in severe cases. Braking distance did not statistically change as tire pressure increased, suggesting that a larger contact patch from underinflation may not be a significant contributor for the conditions explored in these specific tests

Yeah, I'm feeling the same way. I've never done it this way. Every tire is different and, as phantom has stated, have different contact points and need to be inflated to a certain point to achieve the best tire-ground contact. If we inflated all tires to the PSI set on the sticker of the door, some tires would be over/under inflated causing poor traction, poor gas mileage, poor wearing etc.

I was always under the assumption that the sticker on the door was soley for the factory tires, but you know what they say about assuming. ;)

edit: We were posting at the same time. Still doesn't make since to me though. I would think that different tires would need more or less PSI depending on the size and width to obtain proper contact.

i guess you could measure temp distribution along the tire width after a drive or something? to figure out if its over and under inflated

I found this to especially true for the winter tires that I am running ATM. If they are low (2-3psi) I get SLIGHTLY more grip on loose snow but they crap out on ice. If I inflate them properly then you can really feel the studs bite the ice, and I get better fuel economy.

That might work.

This is from a thread over on 6club.

AIR PRESSURE!! - Mazda6 / Atenza

"PLEASE NOTE: THE FOLLOWING INFORMATION WAS OBTAINED FROM TOYO'S ELECTRONIC FITMENT GUIDE. IT MAY BE POSSIBLE TO RUN LOWER PRESSURES THAN STATED BELOW, HOWEVER, PLEASE EXERCISE CAUTION AS IT IS GENERALLY PREFERABLE TO RUN A LITTLE HIGHER PRESSURE THAN TO RUN UNDERINFLATED. IF YOU HAVE SPECIFIC QUESTIONS REGARDING YOUR VEHICLE, PLEASE PM ME. THANKS!

I haven't seen any threads on this, so if I'm repeating info already discussed, I apologize, but it's information worth repeating anyway. I want to bring to everyone's attention the importance of correct air pressure, especially in plus applications (meaning if you have 18 or 19 inch wheels).

When you go to a larger rim diameter, you are making the air chamber smaller, thus requiring additional air in the tire to carry the same load. For example, if your Mazda 6 came with the P215/50R17 tires, your minimum air pressure stated on the door placard is 32 psi. Same is true with the 16 inch tires.

Now, let's say you've upgraded to 235/40R18s. Hopefully, you're using a reinforced construction tire like the Toyo Proxes 4 (load index 95). You'll now need to have at least 33 psi to maintain the same load. Not bad, not a big difference.

But let's say 235s are too wide for you because your wheel offset makes that size rub, so you're using 225/40R18s. If you're using a reinforced size (load index 92), you'll now have to run 38 psi to maintain the same load! Many owners continue to run the original 32 psi and wonder why their tires wear out quickly. Running low air pressure creates heat which accelerates wear on the tire.

If you do not have tires with a load index of at least 91, your tires may not properly carry the load of your car!

Perhaps you've got 19s on your beloved 6. Hopefully, you've got at least a 235/35R19 91V (reinforced). You'll have to run 39 psi to maintain the same load as the oe tire at 32 psi!

Even if you've simply gone to a wider tire on the oe wheels, you need to make sure you've got the right air pressure. A 225/45R17 (94 load index) requires a minimum of 35 psi.

If you've gone to anything smaller than the above listed sizes (like 215/40R18 or 225/35R19) you should probably inflate your tires to the max air pressure listed on the sidewall. Chances are, though, you have a tire that won't carry the load of your vehicle properly.

Also, check your air pressure at least once a month as air will seep out at the rate of 1 psi per month. Your air pressure will also change with the ambient air temperature. For every 10 degrees of ambient air change, your tires will lose/gain 1 psi. If the last time you checked your air was in June and the average temperature was 80 degrees and now its October and now its 60 degrees, you've lost 6 psi of air (4 months + 20 degrees ambient air change)! Make sure you check the air when the tires are cold. Do yourself a favor and get a quality gauge. Thanks for looking!"

My discussion was based on the stock tire size. This is also correct ^^^^

Which is what I felt you were referring to but it seemed that others were getting confused.

If someone does happen to find Toyo's electronic fitment guide that includes recommended tire pressure please post the link.

so?? now im confused....

Yeah, you made it seem like regardless what size tire you ran, you were suppose to utilize the factory sticker.

Thanks for the clarification MS6_Auburn_Fan

edit: My original question to you phantom was " regardless of the brand or size tire you're running, your PSI should be set at what the sticker on the door says? " and you're response was "correct" which apparently is false.

Also the general census: Please correct me if I'm wrong.
To much air = center of tread patch wears faster. Harder ride and more rebound.
To little air = outer of tread patch wears faster. Softer ride (or harder ride if your flat lol) and less rebound. (less cushion from road to wheels, resulting in wheel damage).

when i made my statement i was presuming that we were talking about stock tire sizes. in which case if you use the same size tire on the same size rim, regardless of brand, then go by the information located on the door. the information on the door sticker was designed for stock tire size on the stock rims size based on the weight of the particular vehicle.

Duh! :silly:

running 36-35 all around and its perfect for me.....

I run mine a little harder than you guys do. My summer tires/rims I run 40 front 36 rear, but that's so I don't get roll over in the sidewall on hard cornering. It's all in your driving style and what you're trying to accomplish. Pretty much every track event I've ever gone to, people start at a few PSI under the tires rated max and adjust from there to keep the tire from rolling over.

For winter wheels/rims I run 36 front 34 rear to keep a stable tread pattern, since cornering would be sloppy no matter how much more air I put in them...

This is a good way to sum it up but leave out the portion regarding rebound. Rebound is controlled by the shocks or more appropriate the dampers. While more or less air will change the damping ratio of the tire thus changing the way your car handles it does not change the suspension rebound.

This is true if the same size tire is also the same load rating of the stock tire.

correct

50/50... the sticker on the door is for different size as intended from the factory. Take the regular 3 for example. Tire sizes from the factory are 16, 17, 18 (In Canada). Different sizes yet same sticker on the door. This is because the sticker is for the 18`rim size. Once you go up to a 19 (Over factory size) you need more air than the 18`rim. The 16 and 17 are none consequential due to the fact that they hold more air.

clear as mud.

edit: Think of those road racing bicycles. the tires are insanely skinny and inflated with +100psi (correct me if im worng i`m not sure on the exact number). Then take a truck tire, they are inflated to a lower psi.

The size of the tires have no direct relationship to the air pressure you run. I've run 16's at 50 psi and 18's at 42 psi to get even tread wear out of them on a race course. It has more to do with the weight of the vehicle, load rating, and intent of the driver and tire (on road, off road, straight line, cornering, etc) than it does tire size.

Bike tires have to run high pressures since they have VERY little sidewall structure of their own.

Stock tires, 36psi front 34psi rear, set with a digital guage weekly, wear at
12k seems normal, 1 rotation done a 6500miles.........

Proper tire pressure is critical for safe driving and fuel efficiency, but many passenger and light truck vehicles operate with under or over-inflated tires. Ninety-five per cent (95%) of a vehicle's weight is supported by the tire air pressure, with the tire supporting just 5%, making inflation a critical part of a tire's ability to perform. Tire inflation also has a strong impact on tread life.

Relying on a sight inspection alone is not an accurate way to measure tire pressure. Tires may be significantly under or over-inflated, yet you may not be able to tell just by looking at them.

The only accurate way to know if your tires need to be inflated is by measuring their pressure with a reliable tire gauge. Tire gauges are available at most automotive supply and hardware stores.

* Correct Tire Pressure
Correct tire pressure varies from vehicle to vehicle and wheel to wheel. In fact, the recommended pressure for personal vehicles ranges from 20 to over 50 psi.

The correct tire pressure for your vehicle is listed on the information placard. This placard is normally located on the edge of one of the doors, the inside post of one of vehicle's doors or inside the glove compartment, trunk, or fuel door. Your owner's manual should include the correct tire pressure or direct you to the placard's location on your vehicle.

The pressure listed on the tire sidewall is the maximum tire pressure - or the tire pressure that is required to carry the maximum load of the tire. It is not the manufacturer's recommended tire pressure, which is a common misperception.

In addition to keeping your tires properly inflated, follow these tire maintenance guidelines.

* Under-inflation
Under-inflation is the leading cause of tire failure. Twenty-three per cent (23%) of vehicles have at least one tire under-inflated by more than 20%. Under-inflated tires on your vehicle lead to poor or delayed braking, steering and acceleration. Under-inflated tires may squeal when stopping or cornering even at moderate speeds, particularly on warm pavement.

The Effects of Under Inflation on Tire Wear and Fuel Use
Percentage of Under Inflation Percentage Wear Increase Fuel Use Increase
10% 5% 2%
20% 16% 4%
30% 33% 6%
40% 57% 8%
50% 78% 10%

Operating a vehicle with just one tire under-inflated by 20% (8psi) can reduce the life of the tire by 15,000 km and can increase the vehicle's fuel consumption by 4%. Without enough air, the sides of a tire bend and flex too much. This builds up heat, which can cause serious damage and leads to sudden tire failure. It will also increase rolling resistance, which reduces tread life and increases fuel consumption.

* Over-inflation
Over-inflation can be a problem too. An over-inflated tire rides on just the centre portion of the tread. The smaller contact area means reduced grip on the road, leading to a harsh ride, handling issues (such as steering and stopping problems) and increased wear on tires and suspension components. Seventeen per cent (17%) of vehicles in Canada have at least one tire that is over-inflated by 20%.

Nitrogen Use

Inflating your Tires With Nitrogen
Some retailers are advocating inflating tires with Nitrogen. This may provide certain benefits compared to air inflated tires.

Nitrogen is present as a component of air; it accounts for 78% followed by Oxygen at 21% and other gases at 1% and moisture being a final component of air. Nitrogen in its purest form is a naturally occurring, non flammable, dry, inert gas that can be used for tire inflation without negative environmental impacts.

Over time, air will slowly seep through the tread and sidewall of a tire, resulting in a gradual under inflation of tires, if not periodically topped up. Since a nitrogen molecule is larger than oxygen, it will not seep out as fast as it does with air inflated tires. This means that the gradual decrease in tire inflation pressure with air filled tires (approximately 1 to 2 pounds per square inch (PSI) per month) will be less for tires inflated with nitrogen.

Nitrogen inflation has another potential advantage over air. Nitrogen inflation reduces the amount of moisture, in the form of water vapour, in a tire. The presence of moisture (normally present in air) is the main reason tire inflation pressure varies with temperature. As the air in a tire heats or cools, typically inflation pressure may change by 2 PSI for every 5 degree Celsius change in temperature. This variation can normally be expected to be less for nitrogen inflated tires.

Another potential benefit of reduced levels of moisture in a nitrogen inflated tire is reduced degradation of the tire components over time, caused by oxidation. Normally, the life of a passenger tire is determined by tread wear, rather than aging of the tire, so this may not be a significant factor in many cases.

Consumers interested in nitrogen inflation should evaluate the cost/benefits by consulting a tire specialist.

Who Else Uses Nitrogen?
NASCAR. NASCAR teams use Nitrogen because it allows them to more accurately predict tire pressure fluctuation. Nitrogen fluctuates with temperature change, but it does so less than when water vapour is present. In addition, higher Nitrogen levels eliminate the explosive properties of Oxygen (Oxygen loses its explosive properties at around 9% or less).

Commercial Airlines. The Federal Aviation Administration requires Nitrogen in aircraft tires because it reduces the potential for water vapour freezing at high altitudes. In addition, airlines such as Boeing use Nitrogen membranes or “OBIGGS” on-board inert gas generation systems (OBIGGS) to layer fuel tanks with inert Nitrogen. Again, this reduces the potential for explosions.

Government Agencies. NASA and the U.S. military use Nitrogen for many of the same reasons it's used in commercial aircraft.

Food Processors and Packagers. Oxygen hastens both the chemical breakdown and microbial spoilage of many foods such as meats, potato chips, dairy products and cookies. To help preserve foods longer, processors and packagers often use modified atmosphere packaging (MAP) and controlled atmosphere packaging (CAP) that replaces some or all of the Oxygen in the air inside the
package with Nitrogen.

Industry. Many industries use Nitrogen because of its dry and inert properties. Nitrogen is used in electronics manufacturing to eliminate moisture. It is used in chemical manufacturing to reduce oxidative degradation of the product. It is also used to prevent explosive conditions due to its inert characteristics.

How is Nitrogen separated from other gases in air?
Hollow fibre membrane tubes are the heart of any Nitrogen tire inflation system. Just like tire rubber, the membranes are permeable. When thousands of these permeable tubes are filled with air at high pressures, smaller molecules leak out while the larger Nitrogen molecules travel through the tubes into a holding tank to fill your tires or for other uses such as detecting leaks in air conditioning systems.

Whether your tires are inflated with air or Nitrogen, measuring tire pressure at least “once a month” is critical. Drivers will benefit by increased safety, handling and braking performance, longer tire service life, better fuel economy and reduced greenhouse gas emissions that contribute to climate change and affect air quality

I pulled this from a BMW forum:

Here is couple of spec of Bridgestone Potenza RE 050 that I got from Tirerack website. (***For BMW M5 NOT MS3***)

225/45/17 Max load: 1323lbs Max psi: 51 diameter: 25"
225/40/18 Max load: 1235lbs Max psi: 51 diameter: 25.1"
255/40/18 Max load: 1521lbs Max psi: 51 diameter: 26.1"


If the OEM size were 225/40/17 and recommend psi is 35psi,

[Original Tires Max Load Rating] * [Sticker's Recommended P.S.I.] / [Original Tires Max P.S.I.] * [New Tires Max P.S.I.] / [New Tires Max Load Rating].



225/40/18: 1323 * 35 /51 * 51 / 1235 = 37.49 psi

255/40/18: 1323 * 35 /51 * 51 / 1521 = 30.44 psi


To me Its make more sense if the tire pressure is related to the tire load rating ?

Another little tid bit from another forum:

Ready for a crash-course in the finer points of plus-sizing? Good, lets start with the fundamentals.

The correct way to express your Audi's OEM tire size is depending on the year either "225/45R17 91Y" or "225/45R17 90Y", not "225/45/17". ALL of the information in the tire size is important, not just the physical dimensions, especially when you are talking about plus sizing. You'll see why in a moment.

When plus sizing there are two critical criteria you want to meet.

1. The replacement tires should be as close as possible in overall diameter to the OEM tire size.

2. The replacement tires need to be able to meet or exceed the load carrying capacity of the OEM tires at their recommended inflation pressures. The amount of air the tire can hold determines how much load it can support. To a point the higher the pressure the greater the load. Tires of different sizes have different internal air volumes and sometimes when switching sizes one has to increase air pressure to compensate. There are tables specifically formulated to allow you to do that. Lucky for you I keep one at my computer. This is where knowing the complete size and not just the physical dimensions is critical.

The original 17-inch tires on your Audi have an approximate overall diameter of 25.0 inches and will rotate 833 times per mile when new. The Load Index is either 90 or 91 depending on the year which tells us that at 33 psi the tire can support a load of either 1,235 lbs (90) or 1,268 lbs (91) respectively or at 30 psi 1,146 or 1,179 lbs respectively.

Those are your targets.

A tire sized 235/40R18 91Y or 91W will have an approximate overall diameter of 25.4 inches and rotate 820 times per mile. This is slightly larger than the OEM tires but acceptable. A better choice would have been the size Audi went with as an option on these cars: 225/40R18 XL 92Y. This size has a diameter of 25.1 inches and rotates 830 times per mile - nearly perfect! Anyway, since you have the 235's already we'll run with that.

In a standard loading (235/40R18 91W or 235/40R18 91Y) they have the same Load Index number as the OEM tire = 91. This means at 33 psi this tire carries the identical load of 1,268 lbs and at 30 psi it is 1,179 lbs. Thus, no inflation pressure changes are necessary. You can still run 33/30 with no worries at all. The same holds true even if you have Extra Load (often referred to as "Reinforced") 235/40R18 XL 95W sized tires. At 33/30 psi the load capacity is the same.

Simple, right?

Kind of like how a 19" tire has less side wall than a 18" tire?? Kinda shot yourself in the foot there eh?

Ehhhh, no, the size difference has nothing to do with how the sidewall is constructed. Now, if you would have compared a touring all season tire to a competition road race tire, then maybe you could have pulled it off. Size has nothing to do with pressure, it's how the tire is contructed, how much weight, and intended use that dictate pressure required.

[QUOTE=mordant80;163020]Out to try to prove you're smarter than everyone or what? (Copy and Paste doesn't accomplish that)

No. Not trying to prove myself smarter at all. Just trying to help out. But hey. Whatever.

So what is the best pressure ?

Say what will 30 PSI cause ?

And What will 40 PSI cause ?

I run 32 and 33.5 but now not sure where, but it is from the sticker but on the sticker there is that car +3 people, but I drive 95% with car+one person, but it ould be impractical to pressure them in/out every time I decide to take someone.. Bone stock default tires (dunlop 225/40/R18) on gen2

I'm wondering what is the ideal PSI to get the ideal performance (traction, safety), not to care about MPG (I beleive safety first, then MPG, for God's sake, its MS3, not a prius :) so fuuu the MPG )..

Anyone can help ?

There's a bunch of ways to determine your ideal tire pressure, but unfortunately that's gonna change depending on what you're doing.

The way I do it whenever I change tire types or sizes is use 34/34 cold as a base line. 2 over door plaque in the rear to offset my rear stiffer rear sway bar a bit.

Then I run them fast on the freeway for at least 5 or 10 miles, like 75mph or more if traffic lets me.

Pull off the road and quickly check the now hot air pressure with a gauge and run a pyrometer across the tread. The pyrometer is more valuable for alignment but ultimately we're trying to get a sense of the contact patch while driving so it helps, but you can tell a lot just by hot pressure and looking at the tire over time to see how it's wearing.

Anyway, I just make sure that the pyrometer shows that heat is pretty consistent across the tread. If not, I adjust it as needed - up a pound or two if the center is significantly cooler than the corners of the tread, down a bit if the inverse is true.

Next check, after break in, is to run them in them quickly in the canyons and simply use pressure tests and the little triangle side markers that tell you exactly where the tread ends and the sidewalls begin. With experience I've found that a good tire, driven sensibly, will be in the low 40's in the front and high 30's on a warm (but not blazing hot) day is about right, as long as I'm getting close to the tire marks without going over. Add air if I am, remove it if I'm not.

For track setup, I use both tools and basically just try to get pretty much the same hot pressure from all 4 tires and using all of the tread but no more, and this is different for every track and changes throughout the day quite a bit. Being a FWD car it's normal for the fronts to run a lot hotter in all out driving conditions but the rear tires should be loaded well too, which usually means decreasing more air from the fronts than the rears before hitting the course.

Anyway, regardless of what you're doing, it's generally best just to get the contact patches pretty much equal and watch for wear patterns over time. Rotate every 5K and keep up on wheel alignment. Personally, since I often carry high loads around I expect the rears to be a bit warmer in the middle with just me in the car.

And of course this is largely subjective, especially for a daily driver. Most people want a nice combo setup to get decent ride quality, performance, and life out of the tires. Some people just want higher pressures on the road for sharper steering response, or less from the rear to help the car turn in at low speeds better, etc. And that's fine, but pressure gauges, pyrometers, and lap times don't lie.

BTW, that cold 34/34 baseline I start with? It's been just about always right for me with several different tires and sizes and load ratings. I just tweak it for track use or particular conditions.