Awesome, who else is running 18" wheels in place of 20's

Has anyone figured out the exact size 18" tire to match the diameter of the 168's w/ stock tires? I'm gonna get 177's in both 20's and 18's just need to figure out the tire size to match. Or at least how much tire I can run with out any rub, not to mention thinner > wider when it comes to winter conditions. What's everyone like for 18's