Originally Posted by daisuke149

size of wheel doesnt effect road clearance unless you go too thin of a side wall or too big (too big = more clearance)

But with a 20's the car will feel harder as the tire cant absorb some of the bumps. This also has the effect where too hard of a bump can damage the wheel, where as on the 18's the massive sidewall would have absorbed the bump.

So if you get 20's, as James said, you just have to be more careful and slower.