What is the equivalent size tire 315/35/17 in 18"?

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I am thinking of going to an 18" wheel instead of the 17"s I have now.
My rear tires are 31/35/17 now, I am wondering what the same sized tire would be in an 18?

I could go a little larger possibly, but not much. My tires don't hit or rub now, so I don't want to mess with a good thing