OnStar vs. DIC vs. Pressure Gauge

[klipper]

Quick question:

I've noticed a big difference between what OnStar reports my DIC and a hand held pressure gauge in my front tires.

Hand Held gauge - 29 Psi
DIC - 26 Psi
OnStar - 24 Psi

what do I trust? Does something need to be calibrated with the DIC (cuz I assume OnStar gets its info from that) Does having Nitrogen in the tires have anything to do with this?

[marlboromike]

Do u trust yourself with the hand held gage? That is your answer.

[eddf]

I don't know the specs on the Sky's pressure monitor, but unless it has been proven to be accurate I'd go with a tire gage.

I purchased a digital tire gage for my motorcycle. Made by Accutire. In a couple of reviews it got good marks for being accurate when compared to a calibrated standard. I'll try it this weekend on the Sky and compare readings.

Don't use a tire gage with the white pop-out stick. Very inaccurate.

Ed

[Bogie]

I forgot where I read it, but I believe the TPMS is good to within 1 psi. I have a gauge that matches the TPMS perfectly.

I'm surprised Onstar's report is different than the DIC. I would think the information comes from the same source.

Use a pressure gauge you know is accurate. If the DIC doesn't report within 1 psi, you might want to ask Saturn Tech.

[fr0stb1t3]

My Corvette DIC was always accurate, and that was a much older version of the TPMS. I'd be very surprised if it was wrong in the Sky.

I am betting OnStar's report was taken when the car was not being driven and the tires were cold.

[kennysabarese]

how do you get the DIC to show tire pressure? I have an 07 RedLine and it never gave me a warning when my back right tire was nearly flat.

[ChgoBlues]

Its all dependent on when OnStar actually reads the information. When I drove my RL from Chicago to Boston, I monitored tire pressures since I had to adjust them a bit prior to the trip.

The fluctuation in pressure is very sensitive to ambient temps, how heated the tires are and speed. During my run, the tires went as high as 32psi during upper 70s driving on the highway during the day to 28psi driving at night in the rain.

The TPMS has to be recalibrated if the pressures are adjusted I found out on a trip to my service department prior to the trip.

[TulsaSky]

Originally Posted by kennysabarese

how do you get the DIC to show tire pressure? I have an 07 RedLine and it never gave me a warning when my back right tire was nearly flat.

The 07 cars don't have this feature, it was a government mandate for all 08 vehicles to have the tire pressure monitor systems. Not sure if this can be added to our 07 cars as factory, have to go with a aftermarket system, they are available.

TulsaSky

[klipper]

I thought the whole point of NItrogen filled tires was the absence of pressure fluctuation. That's what the dealer told me anyway...

[fr0stb1t3]

Originally Posted by klipper

I thought the whole point of NItrogen filled tires was the absence of pressure fluctuation. That's what the dealer told me anyway...

Fluctuates less than air, but still fluctuates. I also believe the molecules are larger making it slower to leak.

[mikecronis]

Cold tires early AM undriven vs. warm tires, driven all day make a good difference, maybe up to 5 PSI or more.

[Bogie]

Originally Posted by klipper

I thought the whole point of NItrogen filled tires was the absence of pressure fluctuation. That's what the dealer told me anyway...

As frosty said, it will still fluctuate, but not by much. The big difference in a Nitrogen fill is the reduction in moisture. It's the moisture that is the main cause of fluctuation with a basic air fill, because basic air contains moisture. Even a Nitrogen fill can lead to poor pressure performance if moisture is introduced during the fill (or if existing moisture isn't purged properly).