If you are at the track and running street tires, 20-22 psi. If you are talking about lowering it for every day driving... don't. Keep it at the recommend 32 psi. All lowering it will do is kill your gas mileage.
__________________ 1992 Mustang LX hatchback.. 1 of 542 black on black 2003 Regal GS - the beater/daily driver - somewhat stock. No its not a mustang, at least it's blown.