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Thread: How to check your tire pressure

  1. #1
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    Post How to check your tire pressure

    There is a lot of misinformation here on how to check your tire pressure on this site, so I'd like to set the record straight.

    Some commonly seen misinformation:

    1. You set your pressure based on the number on your tire
    This is false. Very false. The pressure listed on your tire is max pressure cold this is how much air your tire can take without bursting. It is not safe to operate at this PSI UNLESS it is also the recommended pressure.

    2. Set your tire pressure to max PSI - a few lbs
    This is also wrong. Unless that happens to be the recommended pressure this is an over inflated tire.

    3. Over inflated tires are more responsive, give better fuel economy, wear better etc etc etc.
    There is some truth to the first two, but not the third, and actually the third point completely mitigates the second point. An over inflated tire will wear faster in the center, meaning you will need to replace it sooner. This will set back any .x% improvement to fuel economy.

    On responsiveness: yes, an over inflated tire will be a little bit more responsive, but it is also more likely to experience a blow out, and as I have said, won't last as long. If you want something more responsive I suggest you get 18s or 19s. You'll get the same effect in a safer way.

    4. My tires don't look under inflated/have uneven wear so they are fine.
    With modern steal belt radials it takes quite a bit of pressure loss before you can see it. I've put 10psi into my dad's tires before and they looked fine. If you go by wear by the time you find out there is a problem, you already have a tire with uneven wear that will need to be replaced soon. Trust me, a tire pressure gauge is less expensive than a new set of tires.

    5. My car has a tire pressure indicator system, I don't need to check my pressure.
    This one makes me want to bang my head against the wall. The TPI system does not let you know if a tire is improperly inflated, what it does is let you know if there is a significant loss in pressure in one tire vs. the other ones (refer to 4). By the time it comes on, your pressure is already down to the point where your manual would refer to it as "failed".

    Commonly asked questions

    1. Where do you find the tire pressure?
    The tire pressure for your car is on a sticker on your door panel.

    2. How often should you check your pressure?
    Rule of thumb is every other fill up. However, if there is a drastic change in outdoor temperature between fill ups it wouldn't hurt to check them again. Your spare should also be checked regularly. Last thing you want is to go for the spare and find out it is flat. And tires can lose pressure over time.

    3. What is the best gauge to use?
    Circular ones are the best; the ones at the gas station are the worse. Don't depend on those as anything is better than them. Check with another gauge from time to time to make sure yours is accurate.

    4. I have an upsized tire/wheel, what PSI should I have my tire at?
    Find the size closest to your tire and go with that. PSI is a pressure rating, it is pounds per square inch, because of that it isn't a set amount, like a gallon. It will take different amounts of air to get to the correct PSI in differently sized tires, but the right PSI is the right PSI. It is far better than guessing yourself.

    5. When do I check my pressure?
    The best time to check your tire pressure is in the morning before you have driven on it. Tire pressure should always be checked when your tire is cold. Rule of thumb is <2 miles of driving or after it has sat for 2 hours.

    [B]6.What is the big deal about tire inflation?[B]
    Tires are very important to the integrity of your vehicle. A poorly inflated tire can significantly impact your cars handling and braking features and may also lead to a dangerous blow out. This is why it is important to check your pressure often.

    The contact patch of your tire is roughtly the size of your hand. All together, your four tires' contact patch is about the size of a piece of paper. That is all your car is riding on. An under or over inflated tire has a smaller contact patch, which is why performance is impacted.



    As you can see, a properly inflated tire sits flat against the ground allowing for the largest contact patch available.

    Also, because your tire is touching less ground in an over and under inflated tire, those areas are working harder, thus will wear faster. This will mean you need to replace your tires more often, taking hard earned money out of your pocket.

    How to read your door's sticker
    A lot of people accidentally go by the top numbers on their door without realizing that they are over inflating their tires. Here is a picture of the sticker on many of your doors. Yours may vary based on your model. These are just the basics.



    Let’s start at the top. It says Vehicle Capacity | Front | Rear

    "Vehicle Capacity" is where a lot of people get confused. Many go by the top tire pressure without reading that it says max. vehicle weight limit. Unless you have 1,058 lbs of stuff in your car, this pressure is too much. When there is more weight in your car, it pushes the tire down flatter, much like you can flatten a balloon by putting pressure on it. This is why these pressures are higher. But if you set it to that and you don't have 1,000 lbs of people and/or stuff in your car the lack of weight will lead to an over inflated tire.

    "Load up to 4 people" is the second row. This is this is the proper tire pressure for most people in most situations. As you can see, it is 7 and 8 lbs less than the max. inflation. This is a pretty significant difference.

    kPa vs. PSI PSI is the US rating for tire pressure. If you are in a country that uses kPa then use that rating.

    Front vs. Back As you can see your car recommends different pressures in the front and the back. This is for handling reasons. In reality, both are inflated properly. The pressure is different to help enable your car to understeer instead of oversteer. Your car is set up this way because understeer is far safer. You do not need to stagger your PSI. However, if you haven’t had any professional drivers training on a track or during an autocross and don’t quite grasp car dynamics, I suggest you keep it as is. The difference isn’t very significant, but better safe than sorry.

    Shapes Now, this is where you can get confusing. Go by your tire size. And again, if you have an upsized tire go by the one closest to you. As a note, these are the recommended tire sizes of your car. If you are running the right width, but a different height then this tire is sized improperly. It won’t kill you, but it can lead to speedo errors. Tire sizes and their shapes are on the left side next to the chart.

    Cliff notes on other tire information
    1. Checking for damage
    You should also check your tires often for damage. Not all damage leads to a loss of pressure. Something like a tire blister (a circular deformity that looks like a bubble) won't cause you to lose pressure, but is a flaw in the tire that can lead to a blow out.

    If a tire is damaged go to a shop and have them give you their opinion/repair it. Typically, if the damage is to the sidewall it cannot be repaired and you'll have to buy a new tire. It is best to buy two so they are matched in wear.

    2. Tire age
    This is something a lot of people don't think about. But on your tire there is a four number code that represents the manufacturing date. The first two numbers are the week it was made, and the second two are the year. For example, 2508 would be a tire made in the 25th week of 2008. If your tire is more than 6 years old it should be replaced, regardless of wear. Tires are made of organic compounds that decompose over time. They are more prone to blowout and can lead to lower performance.

    Temperature, traction, speed, and treadlife
    Temperature and traction are both rated in A (AA and A for traction) B C lettering. A tire with an AA rating has very good temperature and traction, A has slightly less, B even less. C is the minimum standard for a passenger tire. I would recomm. you stay with at least an A in these areas.

    Speed is rated with letters as well, but they go from Z back. A Z rated tire can go faster than a Y or W rated tire. Typically a tire with a high temperature and traction rating will also have a good speed rating. However, a Z rated tire can go 186 mph safety where as an H rated tire can go 130. So buying a Z rated tire for the sake of it being a Z rated tire is completely useless in your E46

    Treadlife refers to how long the tread lasts. The lower the threadlife, the shorter the life of the tire, but the grippier the tire will be. Now, there are a lot of people on the forums who feel like if they don't track their cars they don't need sticky tires. But grip is grip, whether you are on a road course or the open road. If you are in a bad situation that requires your tires to perform, trust me, that $50 difference is going to seem like a drop in the hat vs the damage that may be caused to your car.

    A tire with a long treadlife will not turn as well and will not stop as well as a tire with a short treadlife. These skills are crucial to car control and I highly recommend you consider them before buying a rock hard tire that will last you 60,000 miles. If your tire makes noise when you make turns, or chirps when you shift, this is your tire’s way of telling you that they are at their limit. If your tires do this in typically low speed situations then think about how that tire would perform at freeway speed. Scary, huh?

    I am also not telling you to go out and buy a tire with a 50 treadlife rating that will last you 5,000 miles. That is over kill and a waste of money. I would, however, recommend a treadwear somewhere between 150-300.

    What tire is best for my car?
    This comes down to personal preference, driving style, driving environment, and budget. But there are definitely good and bad choices for your car. The cheapest thing you can find is a bad choice. It will have super hard rubber and won't have any sort of performance to it. At the same time, a top of the line max performance summer tire would be a bad choice if you live in a place with that stuff you guys call rain or snow. Or if you drive a lot of highway miles. They don't last as long and you might as well roll down your window and throw your money out of it.

    OEM tire is always a good replacement choice. Cars are tuned and designed with this tire in mind, and because of it, the tire usually works well with the car. There are exceptions. The Goodyears they put on Prius's are atrocious and were chosen for quietness, ride quality and fuel economy. None of which lead to a tire with any sort of grip. The contisportcontactII, on the other hand, is a very good tire.

    www.tirerack.com can help you find a tire that fits your needs. Of course, if you live in a place with weather you may want to go with an all season. Those of us lucky enough to never see snow can run a summer tire all year round (note, they typically suck in rain.) I have no experience with snow tires, but I have heard from people here that they recommend that if you get a good amount of snow to have a set of tires/wheels for summer, and a different set for winter with snow tires. The philosophy behind this is: why run a set of tires that performs mediocre in both situations when you can have two sets that perform great?


    For more information on any of these topics and more, please refer to www.tirerack.com's very in debt tire tech section. This is just meant as an overview of important information, not the be all and end all: http://www.tirerack.com/tires/tiretech/tiretech.jsp
    Last edited by SilverBeam; 11-21-2008 at 02:40 AM.

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  2. #2
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    Greta post! This helped me understand that chart a lot better.

    Thanks!

  3. #3
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    Nice post Silver.

  4. #4
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    Perhaps you can answer a problem I have. On the chart what is the REAR tire pressure for 225/45 ZR17? I can't figure it out. Am I just dense? Thanks.

  5. #5
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    Quote Originally Posted by 04e46325i View Post
    Perhaps you can answer a problem I have. On the chart what is the REAR tire pressure for 225/45 ZR17? I can't figure it out. Am I just dense? Thanks.
    I run 225/45/17 and go with 32psi front and 38psi rear.

  6. #6
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    Very Informative. Thank You!

  7. #7
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    Quote Originally Posted by 04e46325i View Post
    Perhaps you can answer a problem I have. On the chart what is the REAR tire pressure for 225/45 ZR17? I can't figure it out. Am I just dense? Thanks.
    Maybe the manual has more information? I don't have an E46 any more so I couldn't confirm.

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  8. #8
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    i run 38 frt 35 rear on my car. i run 38 in the frt due to it helps keep the out side of tire wearing so fast when corning. I have 205-55-16 tires.
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  9. #9
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    You are perfectly safe with whatever pressure you want to run to get the car to handle and ride the way you want as long as it's above the recommended pressure for the car/tire size/load and below the maximum cold pressure for the tire.

    Just make sure you know what you're doing if you're not going to go with the recommended pressures.

    Proper inflation pressure for 225/45-17 on my 2000 328i manual transmission is listed at 44psi for the rear at max load and 36 psi for up to 4 people in the car.

    As for gauges, the best I've found are digital. I have yet to find more than 1 psi variation between the cheapest of the cheap digital pressure gauges and the $250 longacre memory digital gauges.
    Last edited by TeamSlowdotOrg; 11-23-2008 at 07:38 PM.

  10. #10
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    Quote Originally Posted by 04e46325i View Post
    Perhaps you can answer a problem I have. On the chart what is the REAR tire pressure for 225/45 ZR17? I can't figure it out. Am I just dense? Thanks.
    the chart is a little confusing. the rear pressure should be around 38 psi for normal driving, and 46 if you the car loaded up with people and stuff.

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