You can get a free app called Tyresafe, it tells you what the psi should be for your cars' tyres.
If you don’t know the correct pressure for your car’s tyres then look no further than the TyreSafe app. Simply enter your vehicle’s registration details and the app will confirm the vehicle type, tyre size and manufacturer’s recommended pressure settings in both psi and bar.
http://itunes.apple.com/gb/app/tyresafe/id394894885