bkrantz

As always, I reset the tire set once I was done swapping wheels. This should not have any affect on the values measured, just whether the car is wearing a recognized set.

AlBinVA

The first thing I do when leaving the house is to pull up TPMS screen before leaving my street. In my experience, the TPMS are spot on.

Valvefloat991

I just checked this morning and my tire gauge, which is equipped with a replacement expensive industrial gauge head, showed about 31 and 33 front/rear.TPMS showed 28/30.

I also live in Colorado, at an altitude of 5850 feet in Golden. Barometric pressure here is nominally 12.0 psi. Since tire gauges measures differential pressure that suggests that the absolute pressure in the tires would be 43/45 psi front rear

If the TPMS sees those measurements, assumes sea-level pressure, and subtracts 14.7 from them, it would be reading 28.3/30.3, which is exactly what it shows.

I'm a little surprised that Porsche doesn't correct the TPMS readings for ambient pressure. After all, I'm pretty sure the engine measures that parameter to properly calculate the fuel injection.

Another thing I noticed is that as the tires warm up, the TPMS reading sometimes jumps 2 psi at once. I suspect this is because the system's native operation is in tenths of a bar (1.45 psi), and when it converts to psi it does a sloppy job of it.

All in all, I'd rather trust my gauge for precise measurements.

StormRune

Well described! I bet you've nailed it. Like bkrantz noted, there doesn't seem to be any place for the TPMS to take an ambient reading since its only exposure to the outside is the sealed-off valve steam. The car's computer certainly could adjust like you said... its surprising that they don't unless there's some reason I can't think of.

So they already do temperature compensation on the fill pressures. Now I find myself wondering if fill pressures are also compensated for altitude. In other words, if the spot-on setting as shown with your gauge would also result in a zero delta recommendation for fill. Can one or more of you high-fliers check that?

Valvefloat991

I don't think the TPMS pressure readings are temperature compensated. That's why the tire pressure starts out low, when the tires are cold and then rises as you drive and heat the tires.

However, they might be temperature compensated --and perhaps even barometric pressure compensated--on the fill recommendations. I drove my car today and despite the disagreement between the gauge and the TPMS, the fill pressures were 0 to -1 psi. I shall do a more careful check later this weekend.

StormRune

Yes, that's what I said if you check again. I wasn't saying that the absolute pressure reported was temperature-compensated, just the relative fill pressures.

It appears that fill pressures are also altitude-compensated based upon you observations.