How heavy is your car? what size/profile are you tyres? how hard do you drive?
Anywhere between 32-40 psi for road car tyres is acceptable, but optimum depends on the answers to the questions asked above.
Ignore manufacturer recommendations for tyre pressures below 30psi, as they're for bowls-hat wearing Camry drivers who value ride comfort and . . . NOTHING ELSE ! ! !
-Remember the Ford Exploder experience with under-specc'd tyre pressures and radical de-lamination?