Tire Size Comparison Calculator
Compare two tire sizes and see the difference in diameter, circumference, and speedometer effect.
Enter both tire codes to get a full dimensional comparison.
A tire code like 225/55R17 encodes three key dimensions:
- 225: section width in millimeters (the widest point of the tire)
- 55: aspect ratio as a percentage (sidewall height = 55% of section width)
- 17: wheel (rim) diameter in inches
From these three numbers, you can calculate the full geometry:
Sidewall height (mm) = Section Width x (Aspect Ratio / 100) Outer diameter (mm) = (Wheel diameter x 25.4) + (2 x Sidewall height) Circumference (mm) = π x Outer diameter Revolutions per mile = 63,360 inches / (Circumference in inches)
For 225/55R17: Sidewall = 225 × 0.55 = 123.75 mm OD = (17 × 25.4) + (2 × 123.75) = 431.8 + 247.5 = 679.3 mm = 26.7 inches Circumference = π × 679.3 = 2,134 mm = 84.0 inches Revolutions per mile = 63,360 / 84.0 = 754 rev/mile
Plus-sizing means increasing wheel diameter while reducing the aspect ratio to maintain the same overall tire diameter. This preserves the speedometer calibration while allowing a wider wheel. The goal is to keep overall diameter within ±3% of stock to avoid speedometer error and interference with wheel arches.
A 3% diameter difference means the speedometer reads about 3% incorrectly — roughly 2 mph off at 60 mph. Beyond 5% difference, handling and ABS calibration may also be affected because the ABS module uses wheel speed sensors.