What do tyre size numbers mean?

In a tyre size like 205/55R16: 205 is the width in millimetres, 55 is the aspect ratio (sidewall height as a percentage of width), R means radial construction, and 16 is the rim diameter in inches.

Can I fit a different tyre size?

You can change tyre size within limits. The overall diameter should stay within 3% of the original to avoid speedometer errors and clearance issues. Width changes affect handling and may need different offset wheels.

What does the aspect ratio mean?

The aspect ratio is the sidewall height expressed as a percentage of the tyre width. A 55 profile means the sidewall is 55% of the width. Lower profiles give sharper handling but a firmer ride.

What is the R in the tyre size?

R stands for radial construction, which is the standard for modern car tyres. You may occasionally see ZR for high-speed rated tyres. Older bias-ply tyres used B or D, but these are rare on cars today.

What happens if I fit the wrong tyre size?

Fitting incorrect tyres can cause speedometer inaccuracy, altered handling, increased fuel consumption, and potential clearance issues (rubbing on arches or suspension). It may also invalidate your insurance if the tyres are not an approved size for your vehicle. Always check your V5C or door sill sticker for approved sizes.

CarCalcs

Tyre Size Calculator

Decode tyre size markings and calculate the actual dimensions of any tyre.

Width

Tyre width in mm

Aspect Ratio

Sidewall as % of width

Rim Diameter

How We Calculate This

This calculator converts the tyre size code into actual physical dimensions.

The Formulas

Sidewall height = Width x (Aspect ratio / 100)
Overall diameter = Rim diameter in mm + (2 x Sidewall height)
Circumference = Pi x Overall diameter
Revs per mile = 1,609,344 / Circumference

Frequently Asked Questions

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Find out how changing tyre size affects your speedometer reading.

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Last updated: February 2026

All calculations are estimates. Actual figures may vary based on driving conditions, vehicle specifications, and other factors.