Those are stock 17x8" alloy wheels found on deluxe trim 99-04 GT's. From the factory, they were mounted with 245/45r17 tires. That's your best choice for a replacement size.
Any other size that properly fits a 17x8" wheel with throw off your speedometer.
ProCharger P-1SC, 9 psi, STD 396/383; Uncorrected 388/375; SAE 383/370.
USAF 1969-1996: Vietnam veteran. Aircraft maintenance. ICBM Operations. Research & Development.
Also own: 1997 Harley FXDWG, 1998 F-150, and 2002 Corvette LS1