Tire Size Calculator

Compare two tyre sizes side by side showing diameter, sidewall height, circumference, and speedometer error between the original and replacement fitment.

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Original tyre

Width (mm)

Aspect ratio

Rim (inches)

Replacement tyre

Width (mm)

Aspect ratio

Rim (inches)

Comparison

Measurement Original New

Section height 112.8 mm 118.3 mm

Overall diameter 631.9 mm 642.9 mm

Overall diameter 24.88" 25.31"

Circumference 1985.2 mm 2019.7 mm

Impact

Diameter difference +11.0 mm (+1.74%)

Sidewall difference +5.5 mm

Speedo at 100 km/h actual 98.3 km/h

Speedometer error -1.71%

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Tyre size calculator: diameter, sidewall, and speedometer error

A tyre size calculator decodes the three-number format on the tyre sidewall and calculates overall diameter, section height, and circumference. When you change to a different tyre size it also shows the diameter difference and how much the speedometer reading will shift.

Reading the tyre size code

A tyre size such as 205/55 R16 encodes three measurements. The first number (205) is the tyre width in millimetres. The second (55) is the aspect ratio — the sidewall height as a percentage of the tyre width. The third (16) is the rim diameter in inches.

Section height in millimetres equals width multiplied by the aspect ratio as a decimal. Overall diameter equals the rim diameter converted to millimetres plus two section heights — one each side of the rim.

Section height (mm) = Width (mm) × (Aspect ratio / 100)

Sidewall height from the rim edge to the tread surface.

Overall diameter (mm) = Rim (inches) × 25.4 + 2 × Section height

Full tyre diameter including both sidewalls.

Speedometer error with a different tyre size

Your speedometer is calibrated for the original tyre circumference. Fitting a tyre with a larger diameter means the vehicle travels further per wheel revolution than the speedometer expects, so the speedo under-reads actual speed.

The speedometer error percentage is the difference in circumference divided by the new circumference. A change of more than 3% is generally considered significant and can also affect odometer readings and, in some countries, MOT or emissions testing.

Speedo error (%) = ((Original circumference − New circumference) / New circumference) × 100

Positive value means speedo over-reads; negative means it under-reads actual speed.

Frequently asked questions

Can I fit a wider tyre without changing the rim?

Yes, within limits. A wider tyre may rub on the arch or suspension components if the width increase is too large. Most manufacturers allow one or two steps in width (10–20 mm) while keeping the same rim, provided you choose an aspect ratio that keeps the overall diameter close to stock.

Why does the aspect ratio matter for ride quality?

A higher aspect ratio means a taller, more flexible sidewall. This absorbs more road shock and generally gives a softer ride. Low-profile tyres (35 or 40 series) have very little sidewall flex and transmit more road feel and harshness into the cabin.

What diameter difference is acceptable?

Most manufacturers specify that the replacement tyre overall diameter should be within 3% of the original. Beyond that, ABS and stability control systems calibrated for the original diameter can behave incorrectly, and legal speedometer tolerance may be exceeded.

Tire Size Calculator

Tyre size calculator: diameter, sidewall, and speedometer error

Reading the tyre size code

Speedometer error with a different tyre size

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