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Old 06-13-2009, 05:06 PM   #30 (permalink)
Caravanshaka
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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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