bkrantz

Yesterday I swapped my summer tires and wheels for my winter set (the seasons change early in the Colorado mountains). As the final step, I adjusted inflation levels, and used my newish Intercomp tire gauge, with 32 psi front and 40 psi rear, both cold (and my own compromise between comfort pressures and max levels). But when I took my car for a short drive, the onboard pressure readout showed 28 front and 36 rear, a difference of 4 psi.

Once the tires warmed up, the readout showed 30 front and 39 rear. Curious, when I got home I gathered all the gauges I have, from more expensive to cheapish, and got these results:

Front:
35
35
32 (known issue)
35
32 (cheapest)

Rear:
42
43
40 (known issue)
43
41 (cheapest)

If I average the 3 better gauges, it looks like the onboard system reads 4 to 5 psi low. I could more readily accept 1 or 2 psi difference, but this seems significant.

Anyone else see a difference?

StormRune

I've crossed-checked mine (car reading vs tire gauge) a few times and its always seemed to be spot-on. I use a LongAcre liquid-filled gauge that is supposed to be highly accurate.

I'm quite surprised to see your readings so far apart. Out of curiosity, do you know how old the TPMS units in the tires are? At least in the 997 years I know they would start dying at around 5-6 years of age... but that would normally result in no reading... not a bad reading.

erko1905

Newer TPMS are actually decently accurate. I'm usually within 1-2 psi of another gauge I use.

StormRune

After a little thought, I found myself wondering about the altitude where you live. One advantage of the LongAcre gauge I use is that it has a pressure-evening valve that you use prior to taking a reading that allows the gauge to properly "zero" itself based upon the current atmospheric pressure.

Editing since I don't know what the hell I'm talking about here: I wonder if altitude is explaining this in your case? (SW Colorado). Maybe there is something at play between how the TPMS reads pressure at higher altitudes and how tire gauges work at those altitudes. There is a roughly 4psi pressure difference between sea level and 10,000 feet. I dunno. Maybe those of you living up in the thinner air can clarify if altitude might have something to do with this.

skru_fase

Mine are usually within a pound of my Longacre liquid filled gauge too.

Psorcery

Mine are off by a pound compared to the pump/gauge they use at discount tire and I'm OCD as f*** about them. It drives me up the wall when I have non matching pressure. Especially when all 4 are off. My left side always seem to read higher than the right.

bkrantz

I did think about altitude effects, since I live at 7100 ft. Air pressure here is about 3.5 psi less.

Any experts have a good answer?

JW911

Don't you need to reset the dashboard gauge when you put on new wheels so that it synchronizes to the new tpms? I always do as per the owner's manual. Not exactly sure why but I wonder if this will rectify your problem?

Ed911.2

Most tyre gauges that I know of (except the one quoted above) measure differential pressure, i.e. the difference between ambient and the tyre. This means that at sea level, if you have 35psi on the gauge there is actually 50psi in the tyre (~15psi ambient). By the time you get to 10,000’ elevation, the gauge will read 40psi (~10psi ambient) but there is still, all other things being equal, 50psi in the tyre. Temperature normally declines with altitude so will oppose this effect but if it’s warm or you just got the car out of the garage you will see an increase on the gauge.

TPMS *may* compensate for altitude and give you a sea-level referenced pressure, which would account for the differences observed. I’m sure Porsche engineers have thought about this one...

Hurricane

Mine always seem spot on, too, and I use a cheap digital gauge.

CSK 911 C4S

My TMPS is dead nuts on with my little air pumps gauge.

Dennis C

Needsdecaf

Sounds like Porsche TPMS are not compensating for altitude maybe? Remember, what you're reading on the tire gauge pressure, as defined below. Maybe the TPMS measures absolute and then just subtracts 14 psi? Since it has nothing to measure pressure outside the tire, I'd assume that's what it was.

Interesting, next time we take our Cayenne to Colorado, I'll have to watch the TPMS as we start climbing.

• Absolute pressure is zero-referenced against a perfect vacuum, so it is equal to gauge pressure plus atmospheric pressure.
  
• Gauge pressure is zero-referenced against ambient air pressure, so it is equal to absolute pressure minus atmospheric pressure. Negative signs are usually omitted.
  
• Differential pressure is the difference in pressure between two points.
  

Pavegeno928

Yes. Did you go back into the TPMS menu and select winter tires along with your size (19 or 20)? If so, you should have then gotten TPMS learning message and after a few minutes the new pressures will show up. If you didn't do that, you will have errors.

bkrantz

Thanks--that fits my understanding. Since the TPMS sensors are not vented (as far as I know) they measure an absolute value, and cannot compensate for altitude.