I went to check my tire pressure as part of my own post-delivery inspection. They were 32.5 psi all around according to my Campbell Hausfeld digital gauge. My Michelin Digital gauge read 32 psi all around except for 1 tire (not the right rear). The on-board TPMS said every tire was at 31 psi except for the right rear which was 32 psi. WTF?

So I fill up all the tires to 33.0 psi according to my CH gauge. Now my TPMS says all the tires are 32 psi, except the left front is 31 psi. I fill up that tire to 34.0 psi (according to CH gauge) to get the TPMS to show 32 psi.

I hope I didn't confuse anyone, but basically I noticed a significant discrepancy between the on-board TPMS and my handheld gauge. You could argue the handheld gauge is not precise, but it should be consistent. I can get the same reading over and over, so it at least has some precision. In terms of accuracy, that is up for debate but the on-board TPMS doesn't appear reliable. It is off by +/- 2-3 psi at least on my 09 SS/TC.

Any ideas or observations?

 

Well...I have NIST calibrated air pressure gauge (I use them for ISO calibrations for work)...needless to say these are accurate and calibrated annually......The TPMS is off slightly, but its pretty accurate for what it is. My gauge reads 32psi, and the TPMS is reading 30/31psi (changes quickly to 31 once moving).

 

I've come to suspect the accuracy of my sensors as well. So I just check the pressures in the morning, and only peek at the TPMS readings to make sure there's no extreme anomalies.

 

I have experienced this in my 2009 G5 GT. They are within 1-2 lbs and GM says (in the service manual) that the tolerance is +/- 4lbs.

 

I have learned that the air pressure in my tires decreases or increases according to the temperature

 

Thats normal for air

 

The ideal gas law PV = nRT explains that pressure increases as temperature increases. Tires increase in temperature from the flexing of the sidewall, absorption of solar radiation, and friction from road surfaces.

 

Who really cares about 1 or 2 psi??? As soon as you're moving, they will fluctuate anyways. The dashboard gauges are, by far, good enough for me.

 

my right front and right rear are wrong on the tpms. I had a nail in my right rear and it was reading my right front tire being the flat one.

 

It is a great system and that will alert you when there is a problem. Saves the driver the time of walking around the vehicle and inspecting all 4 tires visually and checking the pressures with a gauge at least once a week like a good driver should.

 

hey i learned about that in physics last semester... cool!

 

Sounds like your tires where rotated. There is a procedure in the manual so the computer can relearn which sensor is in each location...

 

Thanks everyone. I guess this is normal behavior but I wanted to check with the other owners first.

 

Mine has been off by a few pounds since day one. I use a manual Tire pressure thing once in a while just to be sure they're ok. My RF one reads WAY low. My RR one reads Way high. Both of my Left side ones are high by like a pound or 2. Sometimes old technology just works better I suppose. I also noticed that when I first start my car they will all be WAY high then by the time I drive 30 miles to work they will be down from the first start by like 4 pounds each tire. I just gave up on the onboard TPMS.

 

They are not accutrate, but accurate enough as an indicator if there is a leak or when one tire is low on pressure compare to others.

 

TPMS is ghey

 

I get the same issue with mine but it is not really a problem. Just get them all to 32-33 lbs with your handheld guage and dont worry so much about what the TPMS reads so much. If it reads 25 psi then I would worry.

 

I believe I read here that the TPMS don't work well until the tire is in motion. I don't know the explanation behind why, but that's just what I heard here.

What I DO know is that the tire should warm up as you drive, thus increasing the tire pressure. This is a very well documented occurrence. For chassis dynamometer testing, a lot of drivability/calibration tests have a warmup procedure that they run in order to get the tires to operating temperature (increase temperature, increase pressure for a fixed volume).

IMO, if the TPMS is within a few PSI, that's good enough. Like I said, pressure varies anyway. If you're within 1 PSI that's as good as you can expect until you get some sort of higher precision, higher $$$ sensors (which is completely unnecessary).

 

You are correct. There are certain temperature and speed conditions that are necessary in order to change the reading.