DIAMETER DIFF +0.38%
Ride height rises by ~1.2 mm
Actual speed at an indicated 100 km/h
Overall diameter (mm) = rim diameter (inches × 25.4) + sidewall height (width × aspect ratio ÷ 100) × 2. For a 205/55R16 tire: 16×25.4 + 205×0.55×2 ≈ 632 mm.
A larger diameter makes the speedometer read lower than your actual speed and slightly raises ride height; a smaller one does the opposite. It also affects odometer and fuel-economy readings and increases the risk of rubbing on the fender or suspension.
A common rule of thumb is to stay within ±3% of the original size. In Japan, inspection requires the actual speed to be 30.9–42.55 km/h at an indicated 40 km/h (vehicles from 2007), so a large diameter change may fail. Rules vary by country.
Keep the overall diameter as close to stock as possible. The classic formula per +1 inch of rim: add 10 mm of width and drop the aspect ratio by 5% (e.g. 205/55R16 → 225/45R17). Use this calculator to check the difference stays within ±3%.
This tool computes theoretical values from the size designation. Real diameters vary by several millimeters between brands and tread patterns, and change with inflation pressure, load and wear. Check the manufacturer catalog or measure for exact values.
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