タイヤサイズ比較計算ツール（インチアップ）

計算の仕組み

Both sizes are parsed from the P-metric code. Sidewall height in millimetres is section width × aspect ÷ 100; overall diameter is twice that plus the rim in inches × 25.4. Circumference is π × diameter, and revolutions per mile are 1,609,344 ÷ circumference in mm. On the defaults, 225/45R17 works out at 634.3 mm and 245/40R18 at 653.2 mm, a +18.9 mm and +2.98% difference, and the table gives you the delta on every derived metric side by side.

Aspect ratio does most of the work, because the sidewall is counted twice. Dropping 225/45R17 to a 40-series costs 22.5 mm of diameter, close to the 25.4 mm a full inch of rim adds, which is exactly why plus-sizing pairs a bigger wheel with a shorter sidewall. Indicated speed and its unit feed only the speedometer line, where true speed = indicated × (diameter B ÷ diameter A); that is why 60 mph on the defaults becomes 61.8. The headline percentage is the number to judge a fitment by.

Everything derives from the printed code, not from a measured tire. Real diameters vary by several millimetres between brands at the same nominal size, and tread wear shrinks them further, so treat roughly a percent as noise. The figures also describe an unloaded tire: under vehicle weight the rolling radius is smaller and true revolutions per mile higher. Only metric sizes containing an R are parsed, so flotation codes are rejected, and clearance is reported as half the diameter change, saying nothing about width.

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