I checked my tyres (Falken ZE-502 - 225's) on the weekend and they dropped about 4 PSI in about 6 weeks.
My mum got her tyres pumped up with urmmm whats it called nitrogen or something (wild guess i have no idea what its called). Cost her like $60 to have it done and mannn their is a difference. Tyres are never REALLY hot, never changed PSI and drives more smoothly.