That's the key, aspect ratio:
think of it as the sidewall height = (aspect ratio) x (width)
both wheels are 20 inch, so you want the same sidewall height on each to achieve the same overall diameter
for a 255/30, height = .30 x 255 = 76.5
for a 305/25, height = .25 x 305 = 76.5
hope that helps
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