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Discussion Starter · #1 ·
I purchased a Jaco digital tire pressure gauge which reads in tenths of a psi and calibrated accuracy of ansi 2a (+-0.5% across full scale). I set all 4 tires to 33.0 cold at sea level in the garage (no sun on any tire). I started the car and waited for the TPMS to register. All 4 tires read 32.0.

I take this to mean that the TPMS reads .8 to 1.2 psi "light" (or the TPMS sensors are only accurate to +- 1 pound). It would make sense that the TPMS would err on the light side since underinflation problems let to the mandate to include this technology (remember the Ford Exploders?).
 

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Or the gauge is off. My stick gauges (and I have a half dozen to a dozen at any given time, depending on how many I've given away to people who've forgotten them on off-roading trips) read about the same as the car. One of my digital gauges is a little off. The other digital gauge agrees with the analog stick gauges and TPMS.
 

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Discussion Starter · #3 ·
Or the gauge is off. My stick gauges (and I have a half dozen to a dozen at any given time, depending on how many I've given away to people who've forgotten them on off-roading trips) read about the same as the car. One of my digital gauges is a little off. The other digital gauge agrees with the analog stick gauges and TPMS.
No, it's the TPMS. This new one was checked by my buddy who does all the track prep for the Porsche racing school and it was right on the money. Actually met the ANSI spec as it was supposed to.
 

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Interesting.
 

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I would trust a calibrated instrument -with the caveat that it's correctly calibrated and within the service interval - more than the (or any, for that matter) TPMS.

Any instrument/device can fall out of calibration.

That said, how much error are we inducing with our sampling? Does how hard we attack the valve stem influence the reading (for example, how a technician loads pipette tips onto a pipettor can significantly influence the measured volume - when you're talking microliters, this is rather problematic....)? etc. and so on?
 

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I would too. I always rely on my gauges. I'm just lucky my TPMS are spot on.
 

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I use this highly calibrated instrument to backup my tpms readings periodically. It’s always been within .5# of the dash reading. I’m happier when the tires all read the same psi on the dash. Is that normal?
Considering my tires’ psi changes a pound or so just sitting in the sun I’m not worried about a precise reading, but having 2 instruments reading similarly is reassuring.
 

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if I remember correctly TPMS only accurate to ± 2-3%. (or ±1 PSI at ~33 ) , as I have TPMS reader and can see 0.1 PSI from sensors, most sensors I have 0.1-0.2PSI from each other and 0.1-0.4 off from my 4 different gauges

OEM sidewall tire pressure over 50+PSI cold ..
door tag 33 cold it is meant to provide the best balance in load handling and ride. etc..

P.S. i prefer to be 1-4+ PSI over 33 compare to 1 psi under 33...
 

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Discussion Starter · #9 ·
I use this highly calibrated instrument to backup my tpms readings periodically. It’s always been within .5# of the dash reading. I’m happier when the tires all read the same psi on the dash. Is that normal?
Considering my tires’ psi changes a pound or so just sitting in the sun I’m not worried about a precise reading, but having 2 instruments reading similarly is reassuring.
Driving in sunny, hot Florida, I have noticed that the tires on the sunny side of the car will heat about .2psi faster than the cool side. After a few miles on the interstate they all stabilize at the same final psi (when the starting psi is equal in all 4).

This is with OEM 20's. Other sizes may behave differently.
 

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^ I see much more variation here in NE-Ohio, particularly in the cooler months.

It's possible that it's also related to how I drive. 😅
 

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compare to 1 psi under 33...
Knowing the load ranges of the OEM 20" tire, -1 would be not optimal for load carrying for those with 33psi door tags.
 
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