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Tire Size Calculator: Compare Two Tire Sizes

This free tire size calculator decodes a size such as 205/55R16 into width, sidewall height, diameter and circumference. Compare it with a second size to see how much your speedometer would be off.

By Debmedia Technologies LLP · Kolkata, India · Updated

What Is It and When Do You Need It?

A tyre size code packs the width, the shape and the wheel size into one string. When you change tyres, a different size changes the overall diameter, which affects the speedometer, gearing and clearance.

Enter your current size, and a second size to compare.

How to Use This Tool

  1. Enter the size in the form 205/55R16.
  2. Optionally enter a second size.
  3. Press Calculate.
  4. Read the diameter, circumference and the difference.

How the Calculation Works

Sidewall = width × aspect ratio ÷ 100. Diameter = 2 × sidewall + wheel diameter × 25.4 mm. Circumference = π × diameter. Revolutions per kilometre = 1,000,000 ÷ circumference in mm. The speedometer error compares the two diameters: real speed = shown speed × new diameter ÷ old diameter.

Worked Example

205/55R16 has a sidewall of 112.8 mm and a diameter of 631.9 mm. 225/45R17 has a diameter of 634.3 mm, 0.4% bigger, so at a shown 100 km/h the real speed is about 100.4 km/h.

Common Uses

  • Choosing replacement tyres.
  • Fitting bigger wheels.
  • Checking speedometer accuracy.
  • Comparing winter and summer sizes.

Tips and Common Mistakes

  • Stay within about 3% of the original diameter.
  • Check load and speed ratings too.
  • Check wheel clearance for wider tyres.

Next, you may need the fuel trip cost calculator, electricity cost calculator, staircase calculator, mortgage calculator.

Everything runs in your browser. Nothing you type is uploaded, stored or sent to any outside service.

Frequently Asked Questions

In 205/55R16, 205 is the width in millimetres, 55 is the sidewall height as a percentage of the width (the aspect ratio), R means radial and 16 is the wheel diameter in inches.
Overall diameter = wheel diameter in mm + 2 × width × aspect ratio.
Most advice is to stay within about 3 percent of the original diameter so the speedometer and ABS keep working accurately.
The speedometer counts wheel turns. A bigger tyre covers more road per turn, so the real speed is higher than shown.

Disclaimer: results are calculated in your browser from the numbers you enter. Check important results before you rely on them for money, health, safety or legal decisions.