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Discussion Starter #1 (Edited)
I have noticed that both the dash display and MyVolt.com often show varying readings for tire pressure when the car is parked. They seem to bounce around a bit, sometimes showing this or that tire low by a pound or two. And of course, when driving it changes and all 4 show the same pressure. Currently parked, the right front and left rear show low 2 lbs.

So it seems that the sensitivity of the tire mounted gauges can be off by up to 2 lbs (5%) or so? I've never had a TPMS before so I don't know if this is typical or not. The manual seems to say, "don't rely on the readings, check the pressure manually" indicating that the TPMS is only a rough estimate, only somewhat accurate.

So when the car is back home I'll get my nice gauge out and see what it shows in comparison.
 

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I see some variation also and ignore it as I check with a gauge once a month and they are all the same and do not leak down much.

Roy #272
 

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It also appears that the left and right are backward on OnStar/MyVolt as noticed by a few. I talked to my advisor and they gave me an OnStar number but keep forgetting to call.

I know this because I keep my left front 1 lb higher than the other 3 to balance the car since I sit in the left front area of the car (Ok this was accidental but really makes a good story I think).
 

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Steverino,

If the car is in the sun the tires on the sunny side can easily be a psi or 2 higher than those on the shady side. The air inside the tires heats up and expands. We see this phenomenon all the time with our race cars where we check the pressure several times a day. On really hot days we try to keep tire covers on the sunny side tires.
 

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Discussion Starter #6
Yes, the Sun Effect occurred to me, but the car points almost true north when parked. So having R front and L rear show low seems counter to what I would expect: both right tires high, left tires low, then changing to rights low, left high as the sun moves. If OnStar/MyVolt is showing mirror image then my left front and right rear are low. I'm going to verify one way or another with a gauge.
 

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Are you saying you are "heavy", Scott200/?
Sort of but keep in my my skeletal bones are big and I know as they were measured during highschool to see how much weight I could lose for my weight class :eek: - Here's the formula for my weight (Volt's battery pack weight / 2)
 

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I have noticed the tire pressure display increase as they heat in the sun as Noel suggested. The sunny side is usually 1 psi higher. I also notice the display shows an increase as I drive at highway speeds, by as much as 2 psi. On cool mornings the display shows less than when I parked it, even though the description says tire pressure "collected during the last drive."

My earlier comment was that all four tires read 2 psi less in either display than what I measure with two different digital tire gauges.
 

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I agree MichaelH. I have used two different guages and got different readings just like you did.

My earlier comment was that all four tires read 2 psi less in either display than what I measure with two different digital tire gauges.
 

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I have noticed the tire pressure display increase as they heat in the sun as Noel suggested. The sunny side is usually 1 psi higher. I also notice the display shows an increase as I drive at highway speeds, by as much as 2 psi. On cool mornings the display shows less than when I parked it, even though the description says tire pressure "collected during the last drive."

My earlier comment was that all four tires read 2 psi less in either display than what I measure with two different digital tire gauges.
Not to restate the obvious, but the tires flex as you drive and create heat which causes the air inside to expand and the pressures to increase. The NASCAR boys always go out after pitstops with the pressure a bit low and the announcers often mention the need to go a bit slower for the first couple of laps until the pressures build up to the optimum. When they cool off over night the pressure goes back down.
 

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When they cool off over night the pressure goes back down.
Yup, Noel. I know the tire pressure is doing what physics requires. The interesting thing is that with the vehicle parked and the pressure changing, the MyLink readout follows the change even though it says, "data collected during the last drive." So it's pretty much real time, unlike MyVolt, which updates tire pressure once per month. So, if you had a slow overnight leak you could see it on MyLink before driving the vehicle.
 

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Yes, the Sun Effect occurred to me, but the car points almost true north when parked. So having R front and L rear show low seems counter to what I would expect: both right tires high, left tires low, then changing to rights low, left high as the sun moves. If OnStar/MyVolt is showing mirror image then my left front and right rear are low. I'm going to verify one way or another with a gauge.
When parked, if the pavement is uneven, some tires can also show more pressure than others and you'll see a variation in PSI. This would normalize out when you're driving.
 

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Discussion Starter #14
After inflating to 36 psi, the car dash reads 34 psi (yes, I drove the car a few miles to allow the sensors to calibrate). So it looks like the sensors are off (low) by 2 psi.
 

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Some advice please. I have the car since May. Today, for the first time ever, I got a low tire pressure warning from the OnStar Remote Link and the MyVolt web app. When I checked with a tire pressure gauge, each tire was a solid 35psi. (the software was telling me 32psi.) The readings, both with the gauge and with the software were done with the car in the garage and the tires cool. Is something malfunctioning? Should I be concerned beyond rechecking every few days?
Thanks,
Stuart #2209
 

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The readings, both with the gauge and with the software were done with the car in the garage and the tires cool. Is something malfunctioning? Should I be concerned beyond rechecking every few days?
My readings from OnStar and my gauge match so I don't have any personal experience. I'd think the question would come down to whether you think it's more probable that four tire sensors are off or whether it's more likely one gauge is off.
 

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My readings from OnStar and my gauge match so I don't have any personal experience. I'd think the question would come down to whether you think it's more probable that four tire sensors are off or whether it's more likely one gauge is off.
By chance I just reset my tires today back to 40-40.5 for all four tires. My car indicates 41 on all four tires and after a few miles of driving in 80+ degree they go to 42. I have a digital gauge that seem accurate.

The first time I did this was was pretty amazed in the swing in tire pressure over a month's time. Up and down like 4 PSI.

P.S. They were set to 40 three months ago and were all down to about 37-38 yesterday.
 

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The first time I did this was was pretty amazed in the swing in tire pressure over a month's time. Up and down like 4 PSI.
You're right that the pressure varies quite a bit. My first thought was that the OnStar measurement and his measurement occurred at different times, accounting for the difference. But if not then I'd think it would be the gauge.

I haven't found a way to force myvolt.com to re-measure the pressure on command. That would be a nice addition and you could make calibrating your gauge easy.

I've been losing a pound or so a month like you.
 

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My tire pressures are off as well.. Initially I got a pressure warning for the front left tire the day after I brought it home. Said it was 32psi. Put the gauge to it and it was 36psi. The other tires said they were 34 and they all were 36 too. I have since bumped all of them up to 45psi, and they now read FL: 42 FR: 44 RL: 44 RR: 45.. Check them with the gauge and they are all 45psi.. so I just accept it as the TPMS sensors are either not calibrated right, or something. As long as the warning light doesn't come on I am fine with it though.
 
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