Porsche Boxster Owner's Manual page 158

Pressure increase as the result of
temperature increase
In accordance with physical principles, the air
pressure changes as the temperature changes.
The tire pressure increases or decreases by
around 1.5 psi (0.1 bar) for every 18 °F (10 °C)
change in temperature.
The Tire Pressure Monitoring takes this relation-
ship between tire pressure and temperature into
account.
156
Instruments, On-Board Computer, Warnings
Tire pressure specifications
Information on tire pressure for public roads can
be found in this Owner's Manual in the Technical
Data chapter or on the tire-pressure plate in the
left door aperture.
These values apply to cold tires at 68 °F (20 °C)
ambient temperature.