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    Dec 16, 2007 11:52:10 AM

    Alex_997TT
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    How accurate is TPMS?

    Just aligned all my wheels whilst cold with my new digital air compressor. I have verified the pressures also with a hand-held digital gauge. This is how I set them up (in psi):

    34 - | - 34

    40 - | - 40

    Interestingly though when I now look at my TPMS readout in the car it shows:

    31 - | - 31

    37 - | - 37

    So -3psi down on the real reading.

    Even more interesting is that when you check the 'info pressure' in the TPMS menu it shows:

    +2 - | - +2

    +2 - | - +2

    Meaning the tyres are over-inflated by 2psi each, which doesn't match with the recommended 34/40 as indicated inside the door.

    So the question is: is this common, or should I take my car back to the dealership to have the TPMS adjusted?

    Dec 16, 2007 1:45:23 PM

    Carlos from SpainM
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    Reply to: Alex_997TT

    Re: How accurate is TPMS?

    Well, one of the two is discalibrated and I would guess its the TMPS given your deltas. I would take it in and show them these numbers.

    Dec 16, 2007 1:59:17 PM

    haudimal
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    Reply to: Alex_997TT

    Re: How accurate is TPMS?

    As said before in the other thread you are right, you have over-inflated by 2psi. The info menu gives you the difference to the norm pressure and takes care of the absolute pressure difference regarding to the temperature.

    Be sure the info shows you +/- 0 and everything is fine, if you choose the right load and tires.

    the absolute pressure is changing all the time but you should only change your pressure when the info menu shows you a +/- value

    more info: http://beru.com/english/produkte/tss.php

    Your dealer can't calibrate anything, they can only change the reference value through PIWIS.

    I wouldn't change the pressure because of different values from an external device.

    Dec 16, 2007 2:02:26 PM

    fritz
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    Reply to: Alex_997TT

    Re: How accurate is TPMS?

    You don't tell us your location, so there are no hints to your current ambient temperatures.

    TPMS control circuitry corrects the pressure readouts in "Info pressure" mode to what the readings would be at 20* Celsius ambient, which explains why these readings do not appear to correspond to the TPMS' own direct pressure readouts when your tires get warm, or very cold.

    Dec 16, 2007 2:31:07 PM

    Carlos from SpainM
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    Reply to: fritz

    Re: How accurate is TPMS?

    Thats a good point, too high/low ambient temperatures could certainly do it. I have found to be as much as 0.2 bar (3 psi?) changes in "cold pressure" values (pressure at ambient temperature after the car has been sitting and not moving) from summer to winter, and my car sleeps inside and the winters here in Spain are mild. So imagine a UK winter and a non-heated garage for example.

    Thats the beauty of the TMPS's "info pressure" display and its delat readings, you will always be spot on no matter the ambient temperature.

    Dec 16, 2007 2:53:19 PM

    Alex_997TT
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    Reply to: fritz

    Re: How accurate is TPMS?

    UK - about 3 or 4 degrees centigrade today.

    This is what is in the driver guide:

    Quote:
    In accordance with physical principals, the air pressure changes as the temperature changes. The tyre pressure increases or decreases by around 0.1bar (1.5psi) for every 10 degrees C (18 degrees F) change in temperature.

    Tyre Pressure Monitor takes this relationship between tyre pressure and temperature into account.



    So surely if I align the tyres (at any ambient temperature) to the recommended 34/40psi (whilst the tyres are cold) then the TPMS should say +0 on all tyres.

    Or are you suggesting that I fill to 34/40, then check TPMS and adjust accordingly up or down?! Surely I want the same tyre pressures when it is 20/10/0 degrees C though?! Or is this what I am missing here?! Do I want a tyre that (when cold) in 20 degrees ambient temperature is the correct pressure, or do I want the correct pressure now?!

    So maybe the -3psi reading difference between real and TPMS could be due to the 1.5psi decrease every 10 degrees from Boyle's law (given it is nearly freezing here).

    Dec 16, 2007 2:54:29 PM

    fritz
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    Reply to: Carlos from SpainM

    Re: How accurate is TPMS?

    Quote:
    Carlos from Spain said:
    Thats the beauty of the TMPS's "info pressure" display and its delat readings, you will always be spot on no matter the ambient temperature.



    The "info pressure" display is the real benefit of TPMS, if you use it to check or adjust your tire pressures whilst the car is stationary.
    The "normal" TPMS display available while driving just has entertainment value until you find yourself with a sudden reduction or loss of pressure.
    Then the display quickly confirms your suspicions.

    Dec 16, 2007 3:08:16 PM

    fritz
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    Reply to: Alex_997TT

    Re: How accurate is TPMS?

    Quote:
    Alex_ said:
    Surely I want the same tyre pressures when it is 20/10/0 degrees C though?! Or is this what I am missing here?! Do I want a tyre that (when cold) in 20 degrees ambient temperature is the correct pressure, or do I want the correct pressure now?!




    You ideally want "correct" pressure at "usual" tire operating temperature. However, the tire operating temperature is higher than either 0* or 20*C, but since one cannot reliably reproduce it when checking tire pressures at a gas station, it is standard practice to quote recommended tire pressures at 20*C - also the international standard temp for calibrating measuring instruments, etc..

    Track cars have lower "cold" pressure settings than road cars, because the pressure increase in normal use on track is greater than for on-road use, due to the heat generated in the tires on the track.

    Dec 16, 2007 3:13:52 PM

    Alex_997TT
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    Reply to: fritz

    Re: How accurate is TPMS?

    I am not sure I understand still...

    Are the tyres pressures MEANT to be lower when the ambient temperature gets colder, or are you supposed to top the pressures up during winter?!

    Dec 16, 2007 3:21:09 PM

    haudimal
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    Reply to: fritz

    Re: How accurate is TPMS?

    Quote:
    fritz said:


    TPMS control circuitry corrects the pressure readouts in "Info pressure" mode to what the readings would be at 20* Celsius ambient, which explains why these readings do not appear to correspond to the TPMS' own direct pressure readouts when your tires get warm, or very cold.



    exactly.

    don't fill in anything in winter unless the "info menu" tells you a "-" value. Regardless what values in the TPMS appear.

    Dec 16, 2007 9:27:53 PM

    xyzx
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    Reply to: Alex_997TT

    Re: How accurate is TPMS?

    yes, they are meant to be lower when the temp is below 20C and higher when the the temp is above 20C. Your car is fine. If pressure is not meant to change, Porsche (and all other manufacturers) would have recommended pressure values without specifying a temprature. But that is not the case. Porsche's recommends pressure values at "20C". Use the TPMS delta display to adjust the pressure of your tires. Adding more air will result in over inflated tires and plenty of vibrations in the cabin.

    Dec 31, 2007 4:14:17 PM

    zeshark
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    Reply to: xyzx

    Re: How accurate is TPMS?

    What about track use ? Isn't TPM a good way to keep hot pressure in check and deflate when tires overheat ? At least indicate whether a manual check is needed after a session ?

    Dec 31, 2007 8:54:05 PM

    Andrew D.
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    Reply to: zeshark

    Re: How accurate is TPMS?

    Quote:
    zeshark said:
    What about track use ? Isn't TPM a good way to keep hot pressure in check and deflate when tires overheat ? At least indicate whether a manual check is needed after a session ?


    My TPMS is far from accurate. Manually check your tire pressures after each run session. Be safe.

    Jan 4, 2008 7:06:19 PM

    Jim48
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    Reply to: fritz

    Re: How accurate is TPMS?

    Quote:
    fritz said:
    Quote:
    Alex_ said:
    Surely I want the same tyre pressures when it is 20/10/0 degrees C though?! Or is this what I am missing here?! Do I want a tyre that (when cold) in 20 degrees ambient temperature is the correct pressure, or do I want the correct pressure now?!




    You ideally want "correct" pressure at "usual" tire operating temperature. However, the tire operating temperature is higher than either 0* or 20*C, but since one cannot reliably reproduce it when checking tire pressures at a gas station, it is standard practice to quote recommended tire pressures at 20*C - also the international standard temp for calibrating measuring instruments, etc..

    Track cars have lower "cold" pressure settings than road cars, because the pressure increase in normal use on track is greater than for on-road use, due to the heat generated in the tires on the track.



    I check my TPMS, automatically, while the tires are warmed up after driving at least 45 - 60 minutes. Then, if they vary too much, I make sure I note the differences between the two front wheels and the two rear wheels and add several tenths of a pound per square inch (we still use the English measurement system in the U.S.). Then I check the TPMS when the tires are cold, just to make sure I am not over-inflating one front (or rear) tire more than the other respective (front or rear) tire. My goal is to have both the front tires equal in pressure and both rear tires equal in pressure, after 45-60 minutes of driving. I end up doing a bit of adjustment each time before I go out for a drive until I have gotten the air pressure correct.

    I think this ensures better, and more predictable, performance, on public roads. I suspect it would be different on a race track, where one might want higher pressures on one side than the other, and that would depend on the specific characteristics of the track and one's driving style.

    Jim

    P.S. I'm glad I purchased the TPMS as an option when ordering my 911S as I bought some cheap insurance for protecting my wheels from damage due to a flat tire, but I also think it was worth it as it helps, in my humble opinion, to improve the handling of the car significantly.

    Jan 4, 2008 10:03:40 PM

    roadtrip
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    Reply to: Alex_997TT

    Re: How accurate is TPMS?

    count me in the unaccurate and for informational purposes only group...there dosent even appear to be much consistancy to the inaccuracy...however it will let you know if you are experiencing a flat which is not a very visible event ... more like a run flat tire flat

    Jan 5, 2008 4:48:55 AM

    Over the Hill
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    Reply to: roadtrip

    Re: How accurate is TPMS?

    This whole discussion is rather confusing to me. I routinely check my tire pressures in the garage before setting out. I adjust pressures to the recommended levels as required. I have three different tire gauges (expensive ones but not digital) that agree with each other and with the TPMS read outs. After driving several miles the tires warm up and the pressures rise a few PSI fron the cold setting. What I have found interesting is, if I park the car with the sun shining on one side of the car the tire pressures on that side will rise significantly. Also during spirited driving I can get tire pressures different from normal at each corner of the car. After driving at a regular pace the pressures return to a normal reading again.

    I guess the points I'm trying to make are it is difficult to use the TPMS to check and adjust pressures on a cold tire. And the pressures vary significantly due to ambient temperatures, exposure and friction during the course of a driving event.

    In the course of checking tire pressures with my tire gauges during a drive the gauge has always agreed with the TPMS readings.

    Jan 5, 2008 10:51:59 AM

    Carlos from SpainM
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    Reply to: Over the Hill

    Re: How accurate is TPMS?

    Quote:
    Over the Hill said:I guess the points I'm trying to make are it is difficult to use the TPMS to check and adjust pressures on a cold tire. And the pressures vary significantly due to ambient temperatures, exposure and friction during the course of a driving event.

    In the course of checking tire pressures with my tire gauges during a drive the gauge has always agreed with the TPMS readings.



    On the contrary, its with TMPS that its easy and imposible to do with a normal gauge

    There is a display you can access withing the On Board Computer menu thats called Tire Presure Info or RDK info or somthing like that (can't remember know the exact wording) that will display the diference in pressure on each tire with the factory pressure INDEPENDANT of the tire temperature.

    I.e. the TMPS measures not only the pressure in the tires but also the temperarure and thus whatever the temperature inside the tires is, it calculates to what pressure it corresponds at the standard 20*C. So the display will show you the difference to the factory pressures measured at the standard 20*C inspite what ever temperature each tire has.

    So when you fill the pressures always use that delta display.

    And for the record my TMPS is always sport on with my digital and analog gauses too

    Jan 5, 2008 4:43:23 PM

    MMD
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    Reply to: Carlos from SpainM

    Re: How accurate is TPMS?

    Download the brochure on calibration:

    http://beru.com/english/produkte/download.php?id=e


    My first TPMS was always off (997S), I never bothered to calibrate it.

    New car is funny because the TPMS is also innacurate BUT it shows correct COLD temps when the tires heat up.

    IOW, if they're 32 when cold the TPMS says 28 (forgot actual numbers); when the tires warm up the TPMS says 32! So I "always" know the cold pressures when the car is hot.

    Not sure if this is the perfect way to use TPMS though... .

    Jan 5, 2008 4:43:52 PM

    SoCal Alan
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    Reply to: Carlos from SpainM

    Re: How accurate is TPMS?

    Quote:
    Carlos from Spain said:
    Quote:
    Over the Hill said:I guess the points I'm trying to make are it is difficult to use the TPMS to check and adjust pressures on a cold tire. And the pressures vary significantly due to ambient temperatures, exposure and friction during the course of a driving event.

    In the course of checking tire pressures with my tire gauges during a drive the gauge has always agreed with the TPMS readings.



    On the contrary, its with TMPS that its easy and imposible to do with a normal gauge

    There is a display you can access withing the On Board Computer menu thats called Tire Presure Info or RDK info or somthing like that (can't remember know the exact wording) that will display the diference in pressure on each tire with the factory pressure INDEPENDANT of the tire temperature.

    I.e. the TMPS measures not only the pressure in the tires but also the temperarure and thus whatever the temperature inside the tires is, it calculates to what pressure it corresponds at the standard 20*C. So the display will show you the difference to the factory pressures measured at the standard 20*C inspite what ever temperature each tire has.

    So when you fill the pressures always use that delta display.

    And for the record my TMPS is always sport on with my digital and analog gauses too



    So when you fill your tires with air, does it give you an audio update of the pressure, or do you have to run back to the driver's seat to check the pressure, run back to give more air, check pressure, run back to give less air, etc ....

    Jan 5, 2008 6:35:34 PM

    jeffbco
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    Reply to: SoCal Alan

    Re: How accurate is TPMS?

    No audio - use a good manual gauge to set the pressures.

    Jan 5, 2008 8:26:56 PM

    fritz
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    Reply to: SoCal Alan

    Re: How accurate is TPMS?

    Quote:
    SoCal Alan said:
    There is a display you can access withing the On Board Computer menu thats called Tire Presure Info or RDK info or somthing like that (can't remember know the exact wording) that will display the diference in pressure on each tire with the factory pressure INDEPENDANT of the tire temperature.

    I.e. the TMPS measures not only the pressure in the tires but also the temperarure and thus whatever the temperature inside the tires is, it calculates to what pressure it corresponds at the standard 20*C. So the display will show you the difference to the factory pressures measured at the standard 20*C inspite what ever temperature each tire has.

    So when you fill the pressures always use that delta display.

    And for the record my TMPS is always sport on with my digital and analog gauses too



    So when you fill your tires with air, does it give you an audio update of the pressure, or do you have to run back to the driver's seat to check the pressure, run back to give more air, check pressure, run back to give less air, etc ....



    The menu item referred to by Carlos is in fact called "TPC: Info pressure".

    After calling it up on the display, on the rare occasions when one or more of the tires has lost pressure, I wind down the door windows and check the readings from the outside of the left and right side of the car in turn after adding some air. Since this is so rarely necessary if you ignore minimum variations of only "-0.1" bar or "-1" psi, it is no hassle. (To explain that last statement, these "minimal" variations might be the result of true corrected pressure dropping from, for instance, 33.51 psi to 33.49 psi, if the cut-off between displayed pressures of 33 and 34 psi is set in the control unit at 33.50 psi).

    Some people seem to be making living with the TPM system unnecessarily difficult for themselves because they have not taken a few minutes to read the relevant notes in the manual and to absorb and understand the concept.

    PS: Just don't ask me to explain why the system is sometimes referred to in English literature as TPC and sometimes as TPMS.

    Jan 6, 2008 12:05:38 AM

    SoCal Alan
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    Reply to: fritz

    Re: How accurate is TPMS?

    Quote:
    fritz said:
    Quote:
    SoCal Alan said:
    Quote:
    Carlos from Spain said:
    There is a display you can access withing the On Board Computer menu thats called Tire Presure Info or RDK info or somthing like that (can't remember know the exact wording) that will display the diference in pressure on each tire with the factory pressure INDEPENDANT of the tire temperature.

    I.e. the TMPS measures not only the pressure in the tires but also the temperarure and thus whatever the temperature inside the tires is, it calculates to what pressure it corresponds at the standard 20*C. So the display will show you the difference to the factory pressures measured at the standard 20*C inspite what ever temperature each tire has.

    So when you fill the pressures always use that delta display.

    And for the record my TMPS is always sport on with my digital and analog gauses too



    So when you fill your tires with air, does it give you an audio update of the pressure, or do you have to run back to the driver's seat to check the pressure, run back to give more air, check pressure, run back to give less air, etc ....



    The menu item referred to by Carlos is in fact called "TPC: Info pressure".

    After calling it up on the display, on the rare occasions when one or more of the tires has lost pressure, I wind down the door windows and check the readings from the outside of the left and right side of the car in turn after adding some air. Since this is so rarely necessary if you ignore minimum variations of only "-0.1" bar or "-1" psi, it is no hassle. (To explain that last statement, these "minimal" variations might be the result of true corrected pressure dropping from, for instance, 33.51 psi to 33.49 psi, if the cut-off between displayed pressures of 33 and 34 psi is set in the control unit at 33.50 psi).

    Some people seem to be making living with the TPM system unnecessarily difficult for themselves because they have not taken a few minutes to read the relevant notes in the manual and to absorb and understand the concept.

    PS: Just don't ask me to explain why the system is sometimes referred to in English literature as TPC and sometimes as TPMS.



    Sorry, Fritz,

    I don't have TPMS, it was not available on 05 997's, so I didn't bother to look at the manual. TPMS sounds like a great feature and I wish I had it.

    Jan 6, 2008 3:01:11 AM

    MMD
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    Reply to: SoCal Alan

    Re: How accurate is TPMS?

    Quote:
    SoCal Alan said:


    I don't have TPMS, [edited]. TPMS sounds like a great feature and I wish I had it.



    Not a problem, do some searching on net. I saw a really nice aftermarket unit made by a reputable company. Not in your face and junky looking. Was about us$200-300. Dammit!: I'm very sorry I can't remember the details.

    Jan 6, 2008 11:02:42 AM

    fritz
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    Reply to: SoCal Alan

    Re: How accurate is TPMS?

    Quote:
    SoCal Alan said:
    Quote:
    fritz said:
    Quote:
    SoCal Alan said:
    Some people seem to be making living with the TPM system unnecessarily difficult for themselves because they have not taken a few minutes to read the relevant notes in the manual and to absorb and understand the concept.




    Sorry, Fritz,

    I don't have TPMS, it was not available on 05 997's, so I didn't bother to look at the manual. TPMS sounds like a great feature and I wish I had it.



    Alan,

    I was not thinking of anyone in particular when I made that comment and certainly not of you, because I took your question to be a genuine query from someone who does not have the the TPMS.

    But the "accuracy" of the TPMS display readings is brought up here with almost monotonous regularity - probably second most overworked topic, right after breaking-in procedures.

    Jan 6, 2008 8:27:07 PM

    Over the Hill
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    Reply to: fritz

    Re: How accurate is TPMS?

    OK now I'm totally confused. I read the referenced TPMS literature, which boggles my mind, so much so that I now have a headache.

    So what I'd like to know from you experts is: If I check and adjust my tire pressures at the current ambient temperature before starting out and recalibrate the system by pressing the TPMS button on the dashboard will my (a) tires be correctly inflated and (b) TPMS instrument light come on if there is a significant loss of air pressure?

    Jan 6, 2008 9:03:32 PM

    fritz
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    Reply to: Over the Hill

    Re: How accurate is TPMS?

    Quote:
    Over the Hill said:
    OK now I'm totally confused. I read the referenced TPMS literature, which boggles my mind, so much so that I now have a headache.

    So what I'd like to know from you experts is: If I check and adjust my tire pressures at the current ambient temperature before starting out and recalibrate the system by pressing the TPMS button on the dashboard will my (a) tires be correctly inflated and (b) TPMS instrument light come on if there is a significant loss of air pressure?



    You cannot "recalibrate the system". There is no button for this purpose on the dashboard. The nominal tire pressures are preset.
    You can only choose between two sets of preset pressures using the on-board computer lever, depending on whether you have fitted summer or winter tires. (There is however some additional provision for allowing for the "Load" (in case of exceptionally high weight of passengers + luggage), but I have never had to use this and have not yet investigated it.

    The tire pressures are correct if you call up the "Info pressure" setting in the display menu when the car is stationary and all values are "0.0 bar" (or whatever equivalent value the psi display shows).

    As I suggested in my earlier post above, one has to resist the urge to become anal if one or more of the four readings in the "Info pressure" mode occasionally shows "+0.1 bar" or "-0.1 bar".

    Jan 6, 2008 9:10:30 PM

    Alex_997TT
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    Reply to: fritz

    Re: How accurate is TPMS?

    As someone who replaced a TPM sensor yesterday...

    I found that after some playing in the TPM menu, if you re-select the tyre type (summer tyres) then TPM goes back into 'learning' mode. It then takes about 10 minutes of driving to pick up the sensors again and then system kicks back in.

    Whether or not it actually recalibates the sensors whilst it is learning I don't know. However, I set the tyre pressures to be 33/39 front/rear from cold at home this morning, then reset the TPM, then went for a drive. When the readings came back up they were 33/39. They were not aligned before I did the reset!!

    Jan 6, 2008 9:14:41 PM

    fritz
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    Reply to: Alex_997TT

    Re: How accurate is TPMS?

    Quote:
    Alex_ said:
    As someone who replaced a TPM sensor yesterday...

    I found that after some playing in the TPM menu, if you re-select the tyre type (summer tyres) then TPM goes back into 'learning' mode. It then takes about 10 minutes of driving to pick up the sensors again and then system kicks back in.

    Whether or not it actually recalibates the sensors whilst it is learning I don't know.



    No, it just relearns how to recognize the transmitters of the individual wheel sensors.

    Jan 6, 2008 9:19:09 PM

    Alex_997TT
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    Reply to: fritz

    Re: How accurate is TPMS?

    Quote:
    Alex_ said:
    Whether or not it actually recalibates the sensors whilst it is learning I don't know. However, I set the tyre pressures to be 33/39 front/rear from cold at home this morning, then reset the TPM, then went for a drive. When the readings came back up they were 33/39. They were not aligned before I did the reset!!



    Quote:
    fritz said:
    No, it just relearns how to recognize the transmitters of the individual wheel sensors.



    Possibly a coincidence then

    Jan 6, 2008 9:21:14 PM

    MMD
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    Reply to: fritz

    Re: How accurate is TPMS?

    Hey Fritz,

    So what you're saying is if TPMS isn't giving you correct readings (after selecting proper summer/winter tire and load in car) THEN that's all you can do?

    I heard somewhere dealer can tweak things to get it "calibrated" correctly; so it will agree with hand held tire pressure guage when cold.

    OR

    Does note seen below (taken from BERU website) mean we can only use factory recommended tire pressures if we _expect_ readings to be "correct" on dash display?
     
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