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  1. #1
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    How can one figure out the "correct" tire pressure?

    I've been having tire balancing issues with my winter tiresicon (17" Hak R's) and so in desperation I went to a dealer yesterday. They got the balancing thing good, but when I checked the TPMS I noticed that the tire pressure was 24f and 31r. Both my owner's manual and the sticker on the driver's side door post tell me the pressure should be 29f and 37r. I called the service fellow first thing this morning and he said that the pressure he set was the correct pressure because of the nature of these new modern tires. He couldn't be more specific than that.
    How do I go about deciding what in fact is the optimal pressure, not only for these tires but for my 18" summer tires too? Inquiring minds want to know.

    TT


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    Re: How can one figure out the "correct" tire pressure?

    Have you checked the pressure manually? Sensors are quite inacurate.
    Tomasz

    Apologies for spelling mistakes, most of them are keyboarding errors. To enjoy this forum I run out of time to proof read.

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    Re: How can one figure out the "correct" tire pressure?

    Actually, I have found that if I'm very careful about setting the pressure manually, the TPMS reflects exactly the pressure I set. It's true that I get odd readings from time to time, and that's led me to wonder if the environment inside each tire has something of a life of its own. For instance, it seems that the wider tires on the rear heat up more than the front tires, making the amount of pressure in the rear tires increase more than the pressure increase in the fronts, maybe because there is more air to heat. But, over-all, I don't have any reason yet to complain about the accuracy of the TPMS.

    TT

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    Re: How can one figure out the "correct" tire pressure?

    The environment inside the tires is different. the TPMS is measuring the air pressure inside the tires. When you use a pressure gauge, you are measuring the difference between the pressure inside the tire vs. the ambient air pressure - then it's converted into the pressure in the tire. You should be measuring your tire pressures with an outside gauge, and not relying on the TPMS. TPMS should really only be used as a system that tells you when a tire is going down quickly because of a leak or damage.

    The winter tiresicon should be lower than the door plate figures because they're 17" and they need to be more pliable in the cold for better traction. When you change back to the 18" summer tires you need to set them to what the door plate or owner's manual says. Set them to those pressures stone cold.

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    Re: How can one figure out the "correct" tire pressure?

    Quote Originally Posted by ThomWT View Post
    I've been having tire balancing issues with my winter tiresicon (17" Hak R's) and so in desperation I went to a dealer yesterday. They got the balancing thing good, but when I checked the TPMS I noticed that the tire pressure was 24f and 31r. Both my owner's manual and the sticker on the driver's side door post tell me the pressure should be 29f and 37r. I called the service fellow first thing this morning and he said that the pressure he set was the correct pressure because of the nature of these new modern tires. He couldn't be more specific than that.
    How do I go about deciding what in fact is the optimal pressure, not only for these tires but for my 18" summer tires too? Inquiring minds want to know.

    TT
    I see a couple of issues here. One is the disagreement between TPMS and a manual gauge, another is what the dealer called the correct pressure versus what the door panel says, and finally you asked what is the optimal pressure.

    First, from what I read on this forum, ignore the discrepancy between the TPMS and the manual gauge. Use an accurate manual gauge to set the pressure. The TPMS is really for monitoring a low pressure or flat tire situation, it's not really designed to be an accurate method to set or monitor pressures. Whenever any shop touches your tire pressure (or lug nuts), always check or re-torque them yourself. They just don't take the time to set them accurately.

    Two, ignore what your dealer said. Dealers and shops typically overpressure tires. If you ask them about it, do you expect them to say "we made a mistake?" No, they will tell you that they did the right thing. This "nature of modern tires" thing is just B.S. to get you off the line. The dealer should, in theory, set the pressure to what the factory specifies. If they don't, any reason they didn't is just an excuse.

    Three, how do you determine what the optimal pressure is. Well, that depends on how you define optimal. The factory recommendation is what they believe is the best compromise between safety (e.g. creating understeer), handling, comfort and mileage. Do you want to optimize mileage? Go slightly higher than the factory recommendation. Optimize the ride? Go slightly lower. Get rid of understeer or optimize handling? Well, now that gets tricky... The best way is on the track with a tire pyrometer. Other than that, change the pressure in one or two pound increments until you dial in the feel and balance (understeer/oversteer) that you like, while monitoring (via chalk or white shoe polish) the wear patterns to make sure you don't roll over onto the sidewall.
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    Re: How can one figure out the "correct" tire pressure?

    Beez,

    I understand that the pressure inside the tires is not the same as the ambient pressure. And I do use a manual gauge. And, as I said, I find that when I get a good consistent reading with my manual gauge the TPMS agrees. What I was trying to convey is the idea that as we drive there are things going on inside the tires which we do not immediately take into consideration which may temporarily affect the TPMS readings. This, at least, is how I try to account for all the little variant and temporary readings I get from the TMPS as I drive. When I start out the TMPS usually reflects the reading I get on my manual. But then in one or another tire the pressure may go up or down a little without any clear correspondence to anything I can identify. I think what I'm trying to say is that what perhaps happens in each individual tire as we drive may be the result of a variety of factors which are not clearly apparent. We may take for granted that all 4 tires are subject to the same influences at the same time, and then conclude that the TPMS is off when it may in fact not be off, but reflecting conditions we don't immediately understand. I hope this is clear.

    TT

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    Re: How can one figure out the "correct" tire pressure?

    Quote Originally Posted by ThomWT View Post
    Beez,

    I understand that the pressure inside the tires is not the same as the ambient pressure. And I do use a manual gauge. And, as I said, I find that when I get a good consistent reading with my manual gauge the TPMS agrees. What I was trying to convey is the idea that as we drive there are things going on inside the tires which we do not immediately take into consideration which may temporarily affect the TPMS readings. This, at least, is how I try to account for all the little variant and temporary readings I get from the TMPS as I drive. When I start out the TMPS usually reflects the reading I get on my manual. But then in one or another tire the pressure may go up or down a little without any clear correspondence to anything I can identify. I think what I'm trying to say is that what perhaps happens in each individual tire as we drive may be the result of a variety of factors which are not clearly apparent. We may take for granted that all 4 tires are subject to the same influences at the same time, and then conclude that the TPMS is off when it may in fact not be off, but reflecting conditions we don't immediately understand. I hope this is clear.

    TT
    Ahhh... I see what you're getting at - and you are correct. As you know, what heats up inside the tires is not only the air molecules, but the moisture that's in the air too. Both expand and tire pressures go up... depending on many factors, pressures can go up in different tires depending on external forces... lots of right hand turns on the way to the store, and the left side tires will go up a few pounds more than the right side tires, for example. Last year at the Porsche Parade autocross, they asked us to pre-grid our cars early in the morning, so I set my pressures cold, then we moved to the next grid, but something happened on-course in the run group before us, and there was an almost hour-long delay... so we got out of the cars and waited. When we were ready to go, I went out to the car, and since it was still in the morning, the bright July 4th sun was shining mostly on the right-hand side of the car - I measured the pressures, and the tires on the right side of the car were now 3 pounds higher than the left side, just from sitting in the sun... so I bled some air out to even things up... So, many things effect how this works. Too keep things even you'd have to be getting out of the car and taking measurements every few miles... which is obviously ridiculous, unless you're tracking or autocrossing. For the street, it's impossible to really keep things precisely even-steven.

    brad
    Last edited by beez; 02-10-2009 at 12:40 PM.
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    Re: How can one figure out the "correct" tire pressure?

    Interesting thread on another site relating to accuracy of tire gauges. Type, age, condition, etc are all factors. Surprising how far off some can be.

    Tire gauges - CarSpace Automotive Forums
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    Re: How can one figure out the "correct" tire pressure?

    Right Beez -- which is why I think that the TPMS is probably more accurate than a lot of people seem to give it credit for, what with all its anomalous and seemingly random readings. Besides that, I suppose that the radio signal the TPMS is sending to the ECU is vulnerable to compromise also--but I'm more doubtful about that.
    But we digress. Since I want to give credit to Porsche for knowing something (not to mention warranty issues), I'm inclined to go with their stipulation 29f and 36r in my case, both for 17" and 18" tires. And then there's the problem of potholes and trying to protect my expensive wheels.
    But on the other hand there is the consideration of flexibility for grip in mucky and slippery conditions (although a tire guy out in Wisconsin or Minnesota with whom I've done business for years tells me that he has found that in ice racing higher pressures seem to be more effective) and fairly significant temperature fluctuations, not to mention wear and gas mileage considerations, and over-all ride comfort.
    The more I write the more I seem to be inclining to the idea that we're all on our own and have to work out the solution that best fits each of us. But I wish there was a way to create a formula that would produce THE answer.

    TT

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    Re: How can one figure out the "correct" tire pressure?

    The notion that the pressure is different inside the tire than in the valve stem where you measure it with a conventional gauge doesn't seem consistent with physics. If there is a difference, then basic physics says that air will flow from a region of higher pressure to a region of lower pressure to equalize the difference.

    The evidence I've seen indicates the TPMS is just not that accurate in an absolute sense. The TPMS in my car sometimes shows pressure differences between the four tires when an external gauge (an Accugage dial type gauge) shows the pressures to be the same. The TPMS also reads 2-3 psi lower, on average, than the gauge.
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    Re: How can one figure out the "correct" tire pressure?

    Car manufactures set recommended tire pressures (OE) based upon the load (weight) the tire needs to carry. When changing tire sizes - plus sizes or minus sizes - it's important that the new tires be inflated to an amount that will at least carry the OE load. The following copied from Toyo Tires website explains this in more detail:


    Basic procedures for reading and applying the load inflation tables:
    Original Equipment
    1. Locate tire information placard to confirm OE tire size and cold inflation pressure. (The tire
    information placard can be found on the vehicle door edge, door jam, glove-box door, or inside of
    the trunk lid.) An example of a T.I.P. is shown in Figure 3.
    Figure 3. Tire information placard
    2. Identify the standard used (TRA for P-metric, LT-metric, and flotation sizes and ETRTO for Eurometric
    sizes) and refer to the appropriate load inflation table. In Figure 3, we recognize that the OE
    size is P225/60R18 (P-metric), so we would refer to the TRA Load Inflation Table.
    3. Find the corresponding load for the OE tire size(s) at the recommended cold inflation pressure.
    Replacement Tire
    4. Use the appropriate load inflation table for the replacement tire size(s).
    5. Find the inflation pressure to which the corresponding load is equal to or greater than the OE tire.
    6. Inflate tires to the appropriate inflation pressure.
    7. If the replacement tire requires a different inflation pressure than OE, the installer should inform
    the owner of the new required inflation pressure and should also place a sticker or decal over the
    vehicle tire placard showing the new tire size and recommended inflation pressure for future
    reference.
    WARNING! Never use an inflation pressure lower than what is recommended by the
    vehicle manufacturer.
    CAUTION! Some vehicles are originally equipped with a staggered fitment in which the
    front and rear tire sizes may vary and/or may have a different front and rear recommended
    inflation pressure. Always maintain any differences in inflation pressures front to rear that
    are shown on the vehicle placard.
    68
    Application of load inflation tables
    Examples of implementing this procedure are carried out in the following:
    P-Metric to Metric
    Example 1. Replace O.E. P235/45ZR17 93W with a Plus-1 245/40ZR18 97W reinforced on a 2006
    Mitsubishi Lancer Evolution IX.
    The O.E. tire is P-metric, therefore use the TRA Load Inflation Table (see Table 3) to look up the load
    capacity at the O.E. inflation pressure. For the standard load P235/45ZR17 93W, at 32 psi the load
    carrying capacity of the front is 1354 lbs and the rear load at 29 psi is 1272 lbs.
    Table 3. TRA Load Inflation Table.
    For a Plus 1 fitment, what should the inflation pressure be if replacing the O.E. tires with a Proxes T1R
    245/40ZR18 97W RD?
    The 245/40ZR18 97W RD is a reinforced ETRTO spec; therefore, refer to the ETRTO Reinforced Load
    Inflation Table (excerpt in Table 4). As indicated previously, always maintain any differences in inflation
    pressures front to rear that are shown on the vehicle placard. In order to maintain the same staggered
    inflation pressure from front to rear, while still carrying an equal or greater load, the front tire must be
    inflated to 35 psi (1378 lbs.) in the front, while the rear tires will need to be inflated to 32 psi (1290 lbs.).
    Table 4. ETRTO Reinforced Load Inflation Table.
    In order to adequately support the load, the 2006 Mitsubishi Lancer Evolution IX with a plus 1 fitment of
    245/40ZR18 97W RD must be inflated to front 35 psi and rear 32 psi.
    26 29 32 35
    93 P235/45R17 1188 1272 1354 1433
    Load
    Index
    Tire Size Inflation Pressure (PSI)
    28 29 30 31 32 33 34 35 36 37 38 39 40 41 42
    97 1146 1190 1246 1262 1290 1334 1359 1378 1433 1454 1477 1521 1547 1565 1609
    Load
    Index
    Inflation Pressure (psi)
    O.E. Information (Obtained from the tire information placard):
    Vehicle: 2006 Mitsubishi Lancer Evolution IX
    Tire Size (Front): P235/45ZR17 93W Inflation Pressure (Front) = 32 psi
    Tire Size (Rear): P235/45ZR17 93W Inflation Pressure (Rear) = 29 psi
    69


    Full information is available at:

    http://marktg.toyotires.com/file/loadinflationtable.pdf

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    Re: How can one figure out the "correct" tire pressure?

    Quote Originally Posted by jake View Post
    The notion that the pressure is different inside the tire than in the valve stem where you measure it with a conventional gauge doesn't seem consistent with physics. If there is a difference, then basic physics says that air will flow from a region of higher pressure to a region of lower pressure to equalize the difference.

    The evidence I've seen indicates the TPMS is just not that accurate in an absolute sense. The TPMS in my car sometimes shows pressure differences between the four tires when an external gauge (an Accugage dial type gauge) shows the pressures to be the same. The TPMS also reads 2-3 psi lower, on average, than the gauge.
    I believe what was being referred to is that the measuring systems (in tire vs external to tire) use differing references.

    For example, if you were to measure the pressure in a totally flat tire, the pressure would be 0 psi referenced to atmosphere. An external gauge has the 'luxury' of referencing the pressure inside the tire to that outside the tire (atmospheric). If they are the same, the tire is for all intents and purposes, flat.

    Now, take a gauge located inside the tire, like most current TPMS systems. When it detects the pressure inside the tire, it cannot reference the pressure outside the tire, so it cannot say for certain the pressure is 0 psi (relative to atmosphere). So, what is done is that the pressure sensing device inside the tire has to utilize a different reference - a vacuum cell, for instance. It can then measure the capacitance across a diaphragm that separates the vacuum cell from the inside tire pressure. This WOULD NOT be the same, even if the tire is totally flat as the pressure inside the tire would not be at vacuum. Therefore, the TPMS system measures against what is essentially an absolute reference (vacuum) vs a variable reference (atmospheric pressure). This alone can introduce variations in pressure indication, unless the external measuring device is calibrated for the immediate conditions.

    Hope this helps.
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    Re: How can one figure out the "correct" tire pressure?

    There are two basic ways to report pressure. One is absolute pressure, which is pressure relative to a vaccum. In this method, ambient atmospheric pressure would be about 14.7 psi at sea level, so a flat tire would have 14.7 psi absolute pressure. The other way is "gauge" pressure, which is absolute pressure minus ambient atmospheric pressure. A flat tire would have 0 psi gauge pressure. Do a Google search and you will turn up many references to these methods.

    TPMS readings are clearly not absolute pressure referenced to a vacuum because they are more like gauge pressure readings. If they are not referenced to true ambient atmospheric pressure, then I don't see how the TPMS readings can be considered accurate measures of gauge pressure either. In my experience, TPMS is useful only as an approximate indication of tire pressure (in a gauge sense) or an early warning of a major problem like a flat.
    Last edited by jake; 02-11-2009 at 08:38 PM.
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    Re: How can one figure out the "correct" tire pressure?

    Just to add here is what a response from Beru (the OEM TPMS provider to Porsche, BMW, Land Rover, Audi, VW and others) said. "The Beru tire pressure monitoring system (TPMS) uses battery-powered wheel electronics which measure the air pressure and temperature inside the tire at short intervals. Together with the individual identification coding of the wheel electronics and information on battery life, these values are transmitted as data telegrams by radio to a radio frequency antenna mounted in the wheel arch. From there they are relayed to the central electronic control unit, The control unit evaluates the data telegrams, identifies the point from which they were transmitted and decides whether the driver should be informed. Each tire is monitored separately. Its air pressure is converted to the standard pressure by means of a temperature characteristic."
    Last edited by Santa Fe; 02-11-2009 at 08:57 PM. Reason: emphasis mine

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    Re: How can one figure out the "correct" tire pressure?

    Quote Originally Posted by jake View Post
    There are two basic ways to report pressure. One is absolute pressure, which is pressure relative to a vaccum. In this method, ambient atmospheric pressure would be about 14.7 psi at sea level, so a flat tire would have 14.7 psi absolute pressure. The other way is "gauge" pressure, which is absolute pressure minus ambient atmospheric pressure. A flat tire would have 0 psi gauge pressure. Do a Google search and you will turn up many references to these methods.

    TPMS readings are clearly not absolute pressure referenced to a vacuum because they are more like gauge pressure readings. If they are not referenced to true ambient atmospheric pressure, then I don't see how the TPMS readings can be considered accurate measures of gauge pressure either. In my experience, TPMS is useful only as an approximate indication of tire pressure (in a gauge sense) or an early warning of a major problem like a flat.
    I'm quite familiar with absolute, atmospheric, gauge, etc pressures, so no reading required there. But, a Google search will also bring up that some TPMS sensors use vacuum as a reference. Not sure what you mean by 'clearly not absolute pressure referenced to a vacuum'. What makes you say that? They would be subject to an internal algorithm, and some may even have temperature compensation, which would have an affect on the reading observed.

    So, I agree, they may not be accurate measures of gauge pressure (or even absolute), but I don't see where that is stated anywhere. Unless the unit qualifiers are psia, psiA, psig, etc, they probably don't pretend to be any pressure precisely. As you state, the intended purpose is to measure pressure delta and warn of low pressure. High accuracy is not a prerequisite in that case. Have you been graphing data in your spare time?
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    Re: How can one figure out the "correct" tire pressure?

    Interesting discussion.

    If the TPMS sensors measures absolute pressure, to get the guage pressure, the system would need to know the atmospheric pressure also. Couldn't that easily be measured by 1 extra pressure sensor to measure atmospheric pressure? Then to obtain the guage pressure, simply subtact the atmospheric pressure reading from the individual tire pressure readings, assuming that the sensors do measure absolute pressure and not guage pressure.

    I would think that if the system did report absolute pressure, it would report a pressure that was about 15psi greater than the manual guage pressure.

    This is of course assuming that the sensors are accurate. I have found mine to be, but who know if that will last over time.

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    Re: How can one figure out the "correct" tire pressure?

    Quote Originally Posted by profpenguin View Post
    Interesting discussion.

    If the TPMS sensors measures absolute pressure, to get the guage pressure, the system would need to know the atmospheric pressure also. Couldn't that easily be measured by 1 extra pressure sensor to measure atmospheric pressure? Then to obtain the guage pressure, simply subtact the atmospheric pressure reading from the individual tire pressure readings, assuming that the sensors do measure absolute pressure and not guage pressure.

    I would think that if the system did report absolute pressure, it would report a pressure that was about 15psi greater than the manual guage pressure.

    This is of course assuming that the sensors are accurate. I have found mine to be, but who know if that will last over time.
    It could pick up the barometric pressure off the engine intake air sensors....or, if it is really looking for a pressure drop, it can just use the absolute pressure without external reference, one would think. I wonder where we could get the proper info on this??
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    Exclamation Re: How can one figure out the "correct" tire pressure?

    Quote Originally Posted by beez View Post
    The winter tiresicon should be lower than the door plate figures because they're 17" and they need to be more pliable in the cold for better traction.
    When you change back to the 18" summer tires you need to set them to what the door plate or owner's manual says. Set them to those pressures stone cold.
    I don't agree with this idea that because they are 17" tires they should be lower pressure. So if they were 16" they should be set at maybe 20 lbs ?
    I think that 24 lbs the dealer used is way too lower to keep a street tire structurally safe. Lower pressure will cause more heat to build up in the tire to the point of destroying it. That's part of the problem Firestone had many years ago which almost put them out of business.

    Porsche calls for lower front, higher back pressures because they partly want to insure under-steer which is safer to the average driver than over-steer.

    I would never go below 29 lbs on any street tire. It doesn't leave much margin for error. If you lose a few lbs and don't realize it, you could start to destroy the tire.

    I do agree that you need to set and check tire pressure in stone cold tire and that the outside temperature will change this by several lbs.
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    Re: How can one figure out the "correct" tire pressure?

    Quote Originally Posted by Augie View Post
    I don't agree with this idea that because they are 17" tires they should be lower pressure. So if they were 16" they should be set at maybe 20 lbs ?
    I think that 24 lbs the dealer used is way too lower to keep a street tire structurally safe. Lower pressure will cause more heat to build up in the tire to the point of destroying it. That's part of the problem Firestone had many years ago which almost put them out of business.

    Porsche calls for lower front, higher back pressures because they partly want to insure under-steer which is safer to the average driver than over-steer.

    I would never go below 29 lbs on any street tire. It doesn't leave much margin for error. If you lose a few lbs and don't realize it, you could start to destroy the tire.

    I do agree that you need to set and check tire pressure in stone cold tire and that the outside temperature will change this by several lbs.
    Support your thoughts. Sidewall design, tread design, rubber compounds, etc would be adjusted to accommodate the needs for a winter tiresicon performance, one would think, to mitigate the need for running relatively lower pressures to attain the needed performance. Isn't this info available somewhere from the manufacturers?
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