Tire Size Calculator

Calculate tire dimensions, convert between metric and imperial, compare two tires side-by-side, or check speedometer accuracy after a tire swap. Step-by-step solutions included.

Calculate

Metric (205/55R16) or Imperial (31x10.5R15)

Choose a calculation type, enter your tire size(s), and click Calculate Now to see dimensions, conversions, or speedometer impact.

Complete User Guide

Understanding Tire Sizes

Tire sizes describe the physical dimensions of a tire — width, aspect ratio (sidewall height as a % of width), and rim diameter. These numbers affect ride comfort, handling, speedometer accuracy, and fuel economy. This calculator works with both metric (P-metric) and imperial (flotation) formats.

Tire Size Formats

Metric (P-Metric)

205/55R16

  • 205 — Width in mm
  • 55 — Aspect ratio (sidewall = 55% of width)
  • R — Radial construction
  • 16 — Rim diameter in inches

Imperial (Flotation)

31x10.5R15

  • 31 — Overall diameter in inches
  • 10.5 — Width in inches
  • R — Radial construction
  • 15 — Rim diameter in inches

Key Formulas

Sidewall = (Width × Aspect Ratio / 100) / 25.4

Diameter = Rim + 2 × Sidewall

Circumference = π × Diameter

Rev/mile = 63,360 / Circumference

Actual Speed = Indicated × (New Dia / Old Dia)

4 Calculation Types

Dimensions

Full specs: width, sidewall, diameter, circumference, rev/mile.

Convert

Metric ↔ Imperial. Nearest standard size.

Compare

Side-by-side diameter, circumference, width, speed impact.

Speedometer

Actual vs indicated speed after changing tire size.

Important Notes

  • Larger tires make speedometer read slower than actual speed.
  • Wider tires improve grip but may rub on fenders.
  • Lower aspect ratio = stiffer ride but sharper handling.
  • Always check your vehicle manufacturer's recommendations.

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