Head over to the Saleen / Roush section and take a poke around. You will find most cars are running 275/35/20"s (front)and 275/40/20"s (rear)on stock Saleen rims. I recommend 18"s (maybe 19"s) all around, and gives you MANY more options for wider wheels/tires and a reasonable tire diameter (especially in the rear).
Front tire size
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