gearing with 18 inch rears

[400EXracer74]

what gears do i change to if i get 18 inch rear tires.

[NacsMXer]

Are you trying to get back to the stock gearing? If so, then you would want to go down about 3 teeth on your rear sprocket seeing that every inch on the rear tires is worth about 1.5 teeth on the rear sprocket. Alternatively, you could go up on the countershaft sprocket one tooth, and down one or two teeth on the rear sprocket to yield similar results.