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.
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