20-inch wheels tires pressures

Just picked up a 2005 Yukon XL Denali, had factory option 20-inch wheels tires (285/55R20). Door jamb doesn't have anything exept for 16/17 inch wheels tires, which it lists at 30 psi. Anybody know what pressures to use for the 20 inch?? Owner's manual doesn't help either. Thanks.