: On board Tire Pressure accuracy



Ste_V_e
12-16-11, 10:56 PM
Hey guys I was just looking at our onboard tire pressure gauge and noticed that at first cold start in the morning the tire pressure reads 31psi for the rears. So I went to a local station that has a machine where you can set the psi and fill the air. After setting it to 32psi I manually checked it with a tire gauge pen and it read aroud 35psi.

I'm wondering how accurate are our on board gauges? Which one do you guys trust and go with? The old fashion way or on board computer? Is 35 psi when the tires are cold too high? I'm just wondering how you guys fill and check your tires.

Thanks guys
-Steve

LV_V
12-17-11, 01:59 AM
I personally wouldn't trust a psi pen over the digital readout. Grab a digital pressure gauge and check it against that.

Gary Wells
12-17-11, 08:30 AM
Both readings might be accurate as the tires would have warmed up during the drive.
I also use a digital to check tire pressure with.

6speeder
12-17-11, 08:31 AM
Only trust the DIC if you are at sea level. The higher your actual altitude, the more the DIC will read low. Ex. here, at 6,000 feet, the DIC indicates 28 PSI when the tires are actually at 32 PSI.

garfin
12-17-11, 09:19 AM
I would only depend on the the car's onboard TPMS to let me know if there was a leak in one of the tires that I was unaware of. I depend on a gauge that I've had for many years to help maintain the pressures I want in the tires.
I've come to the conclusion that the individual tire sensors have minds of their own, since my old dependable gauge will show dead on equal readings while the readings in the DIC shown by the sensors can alll vary somewhat.

Best regards,

Elie

tedcmiller
12-17-11, 02:35 PM
Gary Wells is correct. Any driving to the local station, or otherwise, will cause the tire pressure to increase due to warming.

JFJr
12-17-11, 03:48 PM
My DIC pressure read-out is usually 2 psi below my digital tire gauge reading. I believe that is the margin of error for the on-board system.

elphil
12-17-11, 04:00 PM
On this subject the sticker on the door frame of my Wagon calls for 32 psi front and 35 psi rear. My monthly status email on the Wagon shows a recommended 32 psi all around. I keep it 32 - 35, with a digital gauge.

SecretWeapon
12-17-11, 10:09 PM
+ or - 2 pounds at seal level. Buy a good gauge, check the pressures cold, and use the DIC as an early warning device.

Ste_V_e
12-17-11, 11:48 PM
All really good advice thank you guys. The air machine near my house you can set what psi you want, but i guess they're not very accurate. If I set it at 32psi My gauge pen reads 35 and my DIC reads around 33.

M5eater
12-18-11, 05:47 PM
Which 'digital' gauge is everyone using? What I've noticed is that there are a million different kinds, but none of them acutally quote what their accuracy is. *some* pen gauges state +/- 1-2PSI. I've been assuming no quoted acuracy= +/- 1-2 PSI, the fact that it has a digital read-out or reads in .5 PSI steps does not equal more accuracy or that it's accurate to .5 PSI.

elphil
12-18-11, 10:00 PM
I''m using this gauge: http://www.measurement-ltd.com/12279-det.html

I have an electric pump, similar to the one that comes with the CTS, that you preset the cutoff when it reaches the psi you dial in. I find the cutoff is 5 or 6 psi off. That's why I also use the hand gauge.


.

pmsteinm
12-19-11, 03:26 PM
I find the TPMS is close to my fancy real gauge, but it will read slightly off sometimes because of the way in rounds or something. For example, I have never seen the TPM say "34" (same with the old V)...it goes from 33 to 35. My guess is the TPM don't measure in exactly 1 psi increments.

jnjj
12-19-11, 03:44 PM
I've been using a Moroso 80560 tire gauge for the past couple of months just to see how accurate the TPMS is. There has never been more than +/-1 one pound difference between the gauge and the TPMS. Quite frankly, I'm pleasantly surprised that the TPMS actually works.

DiamondWhtV
12-20-11, 05:56 PM
Mine is accurate with my digital guage on the left side only and shows 1 pound lower on the right. I have no idea why the right side is always 1 pound lower than the left, but I don't worry about it.

GM-4-LIFE
12-21-11, 12:12 AM
Mine is exactly off by 2 psi. My old school gauge will show 32 psi, but the DIC shows 30 psi. Wife's Escalade is exactly the same.

ruby 1
12-21-11, 12:19 AM
Mine is exactly off by 2 psi. My old school gauge will show 32 psi, but the DIC shows 30 psi. Wife's Escalade is exactly the same.
Do you fill the tire to 34psi with school gauge so the DIC reads 32psi?

BeagleBrains
12-26-11, 09:47 AM
The in-car Tire Pressure readings can be calibrated by the dealer. It is most accurate using the pen type extending gauge. Digital gauges just indicate a setting installed by the manufacturer. Also, Digital instrument reading "drift" over time. Get two Digital Pressure Guages; they will give different indications of the same tire!
I worked in a Power Plant for 35 years. We had ten Instrument Techs working every day to verify and adjust deviated Digital Instruments. Electronic devices vary over time.
The direct Tire Valve Stem tire pressure check is the most accurate. Filling with Nitrogen gas gives the most stability due the absence of moisture in the source gas. Nitrogen is provided under more scientific conditions where moisture intrusion is much less likely.
I use an Analogue Pressure Gauge that can be calibrated in the specific range of the required pressure. I have it calibrated about once a year. My right front tire read higher until I had the Dealer calibrate the DIC/TPM against my precisely calibrated reference Analog gauge.
PV nRT is a law of Physics. Pressure changes directly proportional to changes in temperature. Setting tire pressure at 35 pounds will soon produce a "Check Tire Pressure" after a brief period of driving on a summer day, as the tires heat up. Each tire pressure changes independently due the varying effects of turning and road disconformities. The biggest changes occur with major seasonal changes.
http://www.youtube.com/watch?v=WScwPIPqZa0

GM-4-LIFE
12-27-11, 03:19 PM
Do you fill the tire to 34psi with school gauge so the DIC reads 32psi?

No, I rely on my old school guage and not the GM electronics. The car feels perfect at the 32psi reading on my old school guage. At 34 or higher psi on my old school guage, the car rides rough and like a truck. That is how I know my guage is accurate and the car is two psi off.

BeagleBrains
12-28-11, 08:39 AM
Given the condition where you maintain a higher than specified tire pressure, it will reveal itself with a greater center-tread wear pattern. "...guage is accurate and the car is two psi off." It will be especially evident when one tire pressure TPM indication varies from the others, after you have used an old fashioned pressure gauge to set precisely equal pressure in all four tires.
With encroaching winter temperatures, tire pressure will lower. It must be checked and corrected to the required design pressure for each average 10 degree ambient temperature drop.

6speeder
12-28-11, 09:36 AM
The in-car Tire Pressure readings can be calibrated by the dealer. It is most accurate using the pen type extending gauge. Digital gauges just indicate a setting installed by the manufacturer. Also, Digital instrument reading "drift" over time. Get two Digital Pressure Guages; they will give different indications of the same tire!
I worked in a Power Plant for 35 years. We had ten Instrument Techs working every day to verify and adjust deviated Digital Instruments. Electronic devices vary over time.
The direct Tire Valve Stem tire pressure check is the most accurate. Filling with Nitrogen gas gives the most stability due the absence of moisture in the source gas. Nitrogen is provided under more scientific conditions where moisture intrusion is much less likely.
I use an Analogue Pressure Gauge that can be calibrated in the specific range of the required pressure. I have it calibrated about once a year. My right front tire read higher until I had the Dealer calibrate the DIC/TPM against my precisely calibrated reference Analog gauge.
PV nRT is a law of Physics. Pressure changes directly proportional to changes in temperature. Setting tire pressure at 35 pounds will soon produce a "Check Tire Pressure" after a brief period of driving on a summer day, as the tires heat up. Each tire pressure changes independently due the varying effects of turning and road disconformities. The biggest changes occur with major seasonal changes.
http://www.youtube.com/watch?v=WScwPIPqZa0

Interesting that you can calibrate the TPMS at the dealer. Is that with a Tech II?

M5eater
12-28-11, 09:39 AM
The in-car Tire Pressure readings can be calibrated by the dealer. It is most accurate using the pen type extending gauge. Digital gauges just indicate a setting installed by the manufacturer. Also, Digital instrument reading "drift" over time. Get two Digital Pressure Guages; they will give different indications of the same tire!
I worked in a Power Plant for 35 years. We had ten Instrument Techs working every day to verify and adjust deviated Digital Instruments. Electronic devices vary over time.
The direct Tire Valve Stem tire pressure check is the most accurate. Filling with Nitrogen gas gives the most stability due the absence of moisture in the source gas. Nitrogen is provided under more scientific conditions where moisture intrusion is much less likely.
I use an Analogue Pressure Gauge that can be calibrated in the specific range of the required pressure. I have it calibrated about once a year. My right front tire read higher until I had the Dealer calibrate the DIC/TPM against my precisely calibrated reference Analog gauge.
PV nRT is a law of Physics. Pressure changes directly proportional to changes in temperature. Setting tire pressure at 35 pounds will soon produce a "Check Tire Pressure" after a brief period of driving on a summer day, as the tires heat up. Each tire pressure changes independently due the varying effects of turning and road disconformities. The biggest changes occur with major seasonal changes.
http://www.youtube.com/watch?v=WScwPIPqZa0

Could you make a gauge reccomendation?

bash535i
12-30-11, 10:41 PM
I've been using a Moroso 80560 tire gauge for the past couple of months just to see how accurate the TPMS is. There has never been more than +/-1 one pound difference between the gauge and the TPMS. Quite frankly, I'm pleasantly surprised that the TPMS actually works.

Exact same setup and results here. I'd personally put my money on the Moroso being more accurate than most of the digital gauges. The TPMS should be very indicative of the actual pressure.

6speeder
12-31-11, 09:28 AM
Bash and M5: You guys must be near sea level. Here, at altitude, I get reprimanded every month by email that my tire pressure is too low. :(