Join Date: Jul 2007
Location: Anaheim, CA, USA
The TPM is temperature compensated. It isn't perfect, but it is good enough. I have seen minor variations in the TPM readout for a cold tire at a known pressure. The variations are small. They are nearly always +/- 1 PSI, and very rarely +/- 2 PSI. You can assume that the rims are straight, the tire is not worn or damaged, it is mounted properly, the seals are good, the valve is good, etc.
It is reasonable to assume that variations in pressure measurements, whether temperature compensated or not, can be caused by:
* Rounding error (nearest PSI)
* TPM temperature compensation isn't perfect (see bonafidebob's post above)
* Ambient air pressure differences due to weather or altitude changes
* Usage patterns that heat the tire differently (speed, cornering, acceleration...)
* Other environmental factors (humidity inside and outside the tire)
--> The real question is how much over and under the nominal pressure (36 PSI front, 42 PSI rear) are reasonable and safe? For example, would you feel okay at 39 PSI (3 over) for the front tire? What about 33 PSI (3 under)?