Tire Pressure Readings, which to trust?

[Bama]

2015 Cayman S, 75K miles, Michelin tires and stock wheels.

My tire pressure readings using the TPMS is consistently showing 3-4 psi lower than my handheld gauge (I have tried a couple different handhelds). The dealership is telling me to use the TPMS, I think the dealership is incorrect. Has anybody else experienced this?

I am not the original owner of the vehicle and do know the original tires were replaced at some point, however, I do not know if the original TPMS's were replaced.

[blueline]

I have found TPMS readings to be highly accurate, correlating with tire gauges, including excellent ones such as Longacre Racing's analogue dial gauges and cheaper but decent digital gauges. And that experience includes not only Porsches but other makes as well. The TPMS technology and the sensors are fairly well standardized.

However, if your gauge is accurate, then that's what I would trust.

The sensors could be bad - my experience is that they generally have a life of 5 to 8 years. If that doesn't solve the discrepancy then something else must be going on but I don't know what it might be.

By the way, I'm going to delete your duplicate post as I figured it was done unintentionally.

PS - is your display also giving you the same psi "fill" request numbers? Are they the same for each tire?

[blueline]

One thing you might try is to reset your tpms system by changing the tire/wheel type via the settings menu (ie "winter" vs "summer" or a different wheel size - anything) then saving that selection followed by re-selecting your correct tire & rim size and re-saving that. You'll then need to drive until the system reads your pressures, possibly several miles.

Maybe that reset gets everything back in order, assuming the sensors are still good.

[Bama]

Thanks for deleting the unintentional duplicate post, hazard of my job getting distracted.

Yes, the fill numbers are the same. First thing I checked and did a reset. I am going to go with the end of life (assume they were not previously replaced). Due for new tires soon, I'll replace then

Thanks for the input!

[grasmere]

Yes, agree with that, the main purpose of the tpms is the “monitoring” of change,
so if you’ve a ‘good’ gauge then trust that and reset pressures as per @blueline tip.

Perhaps not vital to change the sensors, odd if they ‘all’ are off!

Let us know if you do and how they are, cheers

[Tom]

Are you in a high altitude location? I believe (but have not confirmed) your tire sensors essentially assume they are at sea level -- showing absolute pressure, where ambient is assumed to be the amount of air pressure at sea level. Typical auto parts store tire gauges show the difference between actual ambient pressure and what's inside the tire (I.e. , gauge pressure). Air pressure at sea level is about 14.7. In Denver, for example, air pressure is about 11.7. So, if you put 30psi in your tire according to your pocket tire pressure gauge, that would mean it's 30psi above 11.7, for a total of 41.7psi absolute pressure. Your tire pressure sensors would see that 41.7 absolute and subtract the presumed sea level pressure of 14.7, and display 27psi tire pressure on the dash. This is a common issue on many cars. I have not confirmed it's true on all Porsches, but a possibility to consider, especially if you are in a high altitude area.

[blueline]

To bookend Tom's post regarding altitude, here is a good article and further explanation on subject from Tire Rack:

https://www.tirerack.com/upgrade-garage ... e-pressure

[blueline]

And I agree with @grasmere's comment regarding the oddity of all four sensors going bad at the same time. Anything is possible but that might indicate a different issue?

Then again, maybe replacing all four sensors (it's probably time anyway) along with the corresponding system reset ends up curing the anomaly.

[Bama]

Thanks to grasmere, Tom & blueline. The article was helpful and something I hadn't considered. That said I live in Colorado Springs, roughly 7,000 ft above sea level. I am setting the pressure in COS & autox'g in Denver, about 1,000 below in altitude. Would/could account for some of the (3-4 psi ) difference. Reminder I am only considering cold temp readings. Also agree 4 sensors oddity at the same time would be very unlikely.

I will focus less on the tire pressure and more on the nut behind the wheel while autox. Too many variables for the novice.

[Tom]

Thanks to grasmere, Tom & blueline. The article was helpful and something I hadn't considered. That said I live in Colorado Springs, roughly 7,000 ft above sea level. I am setting the pressure in COS & autox'g in Denver, about 1,000 below in altitude. Would/could account for some of the (3-4 psi ) difference. Reminder I am only considering cold temp readings. Also agree 4 sensors oddity at the same time would be very unlikely.

I will focus less on the tire pressure and more on the nut behind the wheel while autox. Too many variables for the novice.

Standard pressure at sea level is 14.7psia; at 7k feet, that pressure drops to 11.3psia. So your onboard sensors would be 3.4psi off at 7,000 feet compared to a simple tire gauge. Google says pressure in Denver proper is 12.4psia, meaning the sensors would off by 2.3psi vs sea level when measure in Denver. The other big variable is temp, so if you put in X psi before leaving Colorado Springs, then drive to Denver, the tire will be all heated up and the pressure would go up quite a bit but the time you get there. I think the racer's general rule is to rely on the gauge pressure (what you measure with your gauge) when racing, since the tires characteristics will be based on the actual pressure differential inside and outside the tire. Not sure any of that helps, but if it's any comfort, lots of people have dealt with the same thing across a wide range of cars....