305/50/20 vs 305/55/20.. whats the difference in size?

buddy says he could fit a 305/50/20 on his truck without a level kit, but with proper backspacing.

Another question...
What would be equivalent size tires to the sizes above, but with a 18 inch wheel.

 

A 305/55-20 is a 33x12.50 and a 305/50-20 is a 32x12.50. Most manufacturers list a 33x12.50-18 as a 33x12.50 but the metric size would be a 305/60-18.

 

Wandell,
If I wanted to stick with a 18 inch wheel, what tire size would I need, to be close to a 305/55/20 measurement.
Does that make sense???