BFG makes some BFG R1's that fit the 18'' rim .I have some for the track on an extra set of stock fanblades.
Size is 255/40/18 Pretty decent tire for the track only.
i run a 275/40/17 on a set of stock 06 gt bullit wheels, they are tucked because they are about an inch to wide for the wheels but they work fine for me
I e-mailed mickey thompson a couple months back about some 18" drag tires for the stock rims 8.5" and they said that next year they will be coming out with some