Both the manual and the door jamb of the car have tables in/on to tell you laden and unladen pressures of the 205/45/17 Runflats and all other tyres. The manual says 33psi I believe. Thats what I set mine to anyway...I used to run them at 30 psi but the car felt wishy washy and burned through tyres like there was no toimorrow!
Look at the sidewall of your tire, you will (eventually) find its maximum allowable pressure....run them two to three pounds under that for the street if you like a firmish ride.
Of course, reading the manual or the decal on the door arch will give you an insight as well....
Hope this helps
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