What offset should be used on 18's?

 

same as 16's and 17's. between +42 and +48 is ideal.

if you're planning to run 215 width, i would use +45mm.

 

It only depends on the width of the wheel. If you decide to use an 18x7 the offset is different than an 18x7.5 or 18x8. The higher the width of the wheel, the lower the offset needs to be so that you do not make contact with suspension parts. You also have to make sure that the offset will leave enough space on the front side so that it is not sticking out too far...

I would estimate than an 18x7.5 would need between +38 and +42 to fit correctly.

25.4mm = 1"
12.7mm = 1/2"
6.35mm = 1/4"

That may help to give you a visual. More or less, an 18x7.5" wheel with +35 offset will sit "out" 1/2" farther than one with +48 offset.

Drew