Accuracy of TPMS
03-03-2010, 03:30 PM
#1
Instructor
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Accuracy of TPMS
I took my 2007 C4S in to top off the nitrogen in the tires as my dash gauge indicated I was 3# low in each tire. Tire shop added nitrogen but TPMS dash gauge reads 3# less on each tire than the actual pressure as measured at the tire shop with 3 different gauges. I have TPMS set to 'PARTIAL LOAD " which according to my manual is 33 # front and 39 # rear. My dash gauge reads 30# front and 36# rear in spite of tire shop assuring me that the actual pressure is 33# front and39# rear. The trip computer pressure indicator on dash indicates that each tire is 1# low on each wheel except for right front which is 2# low. What gives ? Anyone have similar discrepancies ?
03-03-2010, 05:07 PM
#3
Drifting
The TPMS on my 07 Targa 4S consistently showed 3 lbs less than as measured with tire gauge at each wheel. What I hated was that as soon as the pressure dropped another pound or 2, it would trigger a warning because the TPMS thought it was 4 lbs below proper pressure, not just 1. Then I'd have to over-inflate the tires to get the TPMS to reset, then bleed back to the pressure I wanted.
03-03-2010, 05:39 PM
#4
Rennlist Member
Hmm, mine's around 1 PSI different from tire pressure gauge, but mostly accurate. Never seen that much discrepency.
I've been told it SHOULD be slightly different as the pressure inside is slightly different than on the tire valve.
Make sure you tire gauge is a good one. The ones shops use are always off 2-3 PSI, and most of the gauges sold at Walmart are extremely cheaply made in China crap that are good to tell you if you're under-inflated or not, but not accruate enough for our cars. I bought the Porsche gauge and it looks to be similar to the TPMS, at least in my case.
I've been told it SHOULD be slightly different as the pressure inside is slightly different than on the tire valve.
Make sure you tire gauge is a good one. The ones shops use are always off 2-3 PSI, and most of the gauges sold at Walmart are extremely cheaply made in China crap that are good to tell you if you're under-inflated or not, but not accruate enough for our cars. I bought the Porsche gauge and it looks to be similar to the TPMS, at least in my case.
03-03-2010, 05:41 PM
#5
Drifting
03-03-2010, 06:50 PM
#6
Rennlist Member
03-03-2010, 06:54 PM
#7
Forget about TPMS accuracy (at least for 997.1). It can't be calibrated. If yours is accurate you "won the lottery." 
I'd love to be wrong. If there's a Rennlist Resident Expert out there knows how to adjust them to show the correct pressure PLEASE tell us how to do it!
Waiting......., still waiting........ . LOL
I'd love to be wrong. If there's a Rennlist Resident Expert out there knows how to adjust them to show the correct pressure PLEASE tell us how to do it!
Waiting......., still waiting........ . LOL
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03-03-2010, 07:00 PM
#8
Drifting
It is a shortcoming of the system in the car. Ideally, it should allow you to tell it that whatever signal it is getting from a sensor corresponds to a pressure that you enter (based on your quality tire gauge). But unfortunately, Porsche didn't allow us to calibrate the system. Probably the same guy that designed the Nav. 
03-03-2010, 07:18 PM
#9
Burning Brakes
The TPMS on my 07 Targa 4S consistently showed 3 lbs less than as measured with tire gauge at each wheel. What I hated was that as soon as the pressure dropped another pound or 2, it would trigger a warning because the TPMS thought it was 4 lbs below proper pressure, not just 1. Then I'd have to over-inflate the tires to get the TPMS to reset, then bleed back to the pressure I wanted.
Exact same problem here. BTW, at the risk of starting WW3 or WW4 I hope the nitrogen was free. Otherwise you paid for a lot of hot air and under normal driving conditions in the real world its a waste of $$.
abe
03-03-2010, 07:38 PM
#10
Three Wheelin'
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the TPMS sensors are only accurate to +/- 3psi or +/- 5 psi depending on who you ask.
the justification for the relatively high tolerance is that their intended use is for safety and awareness of leaks, not for precise measurement of the tire pressures.
don't shoot the messenger - i didn't spec the things.
the justification for the relatively high tolerance is that their intended use is for safety and awareness of leaks, not for precise measurement of the tire pressures.
don't shoot the messenger - i didn't spec the things.
03-03-2010, 08:13 PM
#11
03-03-2010, 09:14 PM
#12
Drifting
Mine shows 3 psi below actual and the speedo shows 3mph faster than actual. Hmm wonder if there is a hidden meaning here.....
03-03-2010, 09:40 PM
#13
Three Wheelin'
Somehow they must have gotten it sorted out in the ".2" - mine is very close +/- < 1lb with the Accutire.
03-03-2010, 10:14 PM
#14
Drifting
No two air gauges ever agree. The 6 odd gauges I have laying around the garage disagree by 15psi! The three I use most of the time are 3psi apart, so I assume they're at least in the ballpark, but I don't know which one is closest. Honestly, though, it doesn't really matter. If that gauge is off by a little, it's no big deal- neither you or your tires will notice 2psi of difference. Hard to even measure it on the skidpad. It'll vary by that much in the course of a drive as the tires heat anyhow.
So, if TPMS and your tire gauge do or don't agree... they're probably both wrong anyhow. Best you can really do is check with a couple gauges so you know you're close, and try to keep consistent with which gauge you adjust pressure with.
So, if TPMS and your tire gauge do or don't agree... they're probably both wrong anyhow. Best you can really do is check with a couple gauges so you know you're close, and try to keep consistent with which gauge you adjust pressure with.
03-03-2010, 10:56 PM
#15
BTW, you guys make an excellent point. 
I think I did this a while ago when I first was bothered by TPMS discrepancy. I'll do it again since I don't remember the details except to have become disappointed with TPMS.
I have three gauges, all non-cheap ones, in different cars, will try to test them all tomorrow or the next day. Got a feeling they will agree with each other and disagree with the TPMS. Why? because they're all electronic, two different mfgrs, two from same mfgr bought a year apart.
If they do disagree, I'll put the one that matches or closely matches the TPMS in the P-car!
Will report back.
I think I did this a while ago when I first was bothered by TPMS discrepancy. I'll do it again since I don't remember the details except to have become disappointed with TPMS.
I have three gauges, all non-cheap ones, in different cars, will try to test them all tomorrow or the next day. Got a feeling they will agree with each other and disagree with the TPMS. Why? because they're all electronic, two different mfgrs, two from same mfgr bought a year apart.
If they do disagree, I'll put the one that matches or closely matches the TPMS in the P-car!
Will report back.