40psi is to high for most vehicles.
Here is a guide.
Smaller cars Micra, Swift etc. 34-36psi
Medium cars Commodore, falcons etc. 36-38psi
4WD's sit good at around 40psi
Trailer tyres and light truck tyres at the Recommended PSI anywhere from 40-65psi.
This is the pressures we set them at...