Enter two sizes (like 205/55R16) to see the overall diameter, the difference, and how far your speedometer will read off.
A tire size like 205/55 R16 encodes three numbers: section width (205 mm), aspect ratio (55 — the sidewall height as a percentage of width) and rim diameter (16 inches). The overall diameter is the rim plus twice the sidewall height. This tire size calculator works that out for two sizes and compares them: the diameter difference, the change in circumference, and the resulting speedometer error.
When "plus sizing" (bigger rim, lower profile) you normally keep the overall diameter close to stock — within about 3% — so the speedometer stays accurate, the ABS/traction systems behave, and there's no rubbing. A bigger diameter makes the speedo under-read (you're going faster than it says); a smaller one makes it over-read.
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