[quote=Minok;2498427]So can someone explain why the diameter of the wheel would make any difference in the tire pressure reading from the TPMS sensor?

I understand that for the system to tell you what the right pressure is - ie the display of the differential pressure form where it should be would need to know the size of the tire.

But the pressure inside the tire is unrelated to the size of the tire/wheel. Its an absolute number and its the same number on any make and model car or tire or wheel........."

Not to be long winded, but the original TPMS were 'direct' whereby sensors inside the wheels measured the pressure. Many of the latest systems, including the ones on pcars, are what are called 'indirect'.
Indirect TPMS do not use physical pressure sensors. They measure the "apparent" air pressure, by monitoring individual wheel rotational speeds, and other signals available outside the tire itself. Most indirect TPMS use the fact that an under-inflated tire has a slightly smaller diameter than a correctly inflated tire and therefore has to rotate at a higher speed to compensate. Most current advanced indirect systems use the spectral content of the wheel speed sensor signals so no additional sensors are needed and the computations can be carried out by usual processors in the ABS or ESC control units. All of this is part of our PSM system.
If you want more talk to your dealer's service manager or go to Wikipedia.org.

So......as some have already posted, the purpose of tpms is to show pressure drops, not to give an accurate real time reading

 

Hate to disagree with fellow board member, but tejoe, I don't believe that information is correct. Cars like the E46 M3 have the tire pressure "monitors" that track pressure through the rolling size of the tires (that's why aftermarket tires can make the things go off even after you calibrate them). P cars now have TPMS with actual transponders in the wheels that actually measure the tire pressure. That's why they're so damn expensive - over $350 per set. THey should actually convey an accurate pressure reading to the Porsche computer. My question goes back to wondering if those readings need to be "calibrated", i.e. the pressure reading from the sensor coded into the computer with the correct number to establish a "zero" reading for the TPMS to report pressure information. I'm getting a set of new wheels with new senders so I guess I'll learn about this painful process soon myself.

 

My TPMS indicates different pressure when the car is not moving - i.e., when I park it vs when I go out the next day to start it or after I add air. I don't think rotational speed, ABS, or ESC can account for that.

This seems like a fair start at an explanation: https://www.6speedonline.com/forums/...formation.html

 

Cool link. Exactly as I said. Unfortunately doesn't answer the question of calibration...

 

+1 off by 4 psi. i run 30 36 with cup tires. the TPMS sucks. I reset it and is still throws a CEL

 

As I posted in another thread on this issue, I found out that when I got the warning, it was because in the TPMS menu, my car was set for a "full" passenger load. When I reset it for a "partial" passenger load, my TPMS is reading perfectly now.

 

I just got one of those Accutire gauges - turns out my little mechanical one was working pretty well too. My TPMS system is reading really close to what the manual says the tires should be and within a pound of the accutire meter.

As Joe mentions, very important to have the settings in the function menu correct or it will throw you off by 4 or 5 lbs.

For a setup like that, I think there is nothing in the function menu to handle that case; only 18 or 19 summer tires, and partial or full load. Ranges for fronts are in the mid to upper 30s and rears are high 30s to low-mid 40s.

 

If the TPMS does an indirect measurement taking into account rim diam and speed... when someone installs a 20" rim which is not on the list (18"or 19" only) a TPMS reading discrepancy is to be expected, isn't it?

 

Exactly.

The 997.2 (in my case) does not have a place for me to select 18" or 19" wheel. It could be the dealer selects that option by the PIWIS machine, but that would be nuts, if one was swapping summer/winter and they were 19" summers and 18" winters. I'm suspecting the car's sensor-recievers can determine the size of the wheel, and all you tell it is 18" or 19".

However, knowing the wheel size and if its summer or winter is great for telling you if your tire pressure is above or below the recommended pressure for that combo. But what about just plain basic pressure value? Thats the same no matter what. 40psi is 40psi, no mater what wheel or tire it is on, and I'd expect a TPMS sensor to be able to give me a PSI reading without having to know the tire type and wheel size.

But since I don't know the exact electronics mechanism used to sense the pressure in the tire and then transmit that knowledget to the car, I assume it might be possible that the data values the PCM reads depend on the proper positioning of the TPMS sensors to the appropriate receivers in the car, and that a 20" rim just will not transmit clear readings.