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I would not personally put 245s on 7.5 width rims. That aside, the tire diameter would be 27.7. That is pretty tall for most Lexus. What car is this going yp be used for?
I think the 7.5 is a little thin for the GS. 8.5 would be a better size. If the rims are set, then 225/40 would be good for a 7.5 although I would push it and go 235/40 to be closer to OEM spec. If you had 8.5 (or even 8), 245/40 would be ideal.