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Tyre Size Calculator

Calculate dimensions of car tyres (sidewall height, overall diameter, circumference) and compare size upgrades to avoid speedometer error.

mm
135 mm335 mm
%
30%85%
"
13"24"
Calculation Results
Overall Tyre Diameter
632 mm
Equivalent to 24.88 inches
Sidewall Height
112.8 mm
Rolling Circumference
1985.5 mm
Revolutions Per Km
504 revs/km
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How to use the Tyre Size Calculator

  1. 1Enter section width in millimeters (e.g. 205).
  2. 2Enter aspect ratio profile (e.g. 55).
  3. 3Enter rim diameter in inches (e.g. 16).
  4. 4Optionally enter stock tyre diameter to check speedometer calibration differences.

How Tyre Size is Calculated

Sidewall Height = Width × (Aspect Ratio / 100). Overall Diameter = (Rim Diameter × 25.4) + 2 × Sidewall Height.

Mathematical Formula
Sidewall (mm) = Width × (Aspect / 100) Diameter (mm) = (Rim × 25.4) + 2 × Sidewall Circumference = π × Diameter
Formula Variables & Logic

Speedometer error occurs when replacement tyres have a rolling circumference that differs from factory stock.

Worked Example: Calculating Dimensions of 205/55 R16 Tyre
Sample Calculation
Scenario: A standard passenger tyre sized 205/55 R16.
Step-by-step breakdown
  1. 1Sidewall Height = 205 × 0.55 = 112.8 mm
  2. 2Rim Diameter in mm = 16 × 25.4 = 406.4 mm
  3. 3Overall Diameter = 406.4 + (2 × 112.8) = 632.0 mm (24.88 inches)
  4. 4Circumference = π × 632.0 = 1,985.5 mm
Estimated Result
Overall diameter is 632.0 mm (24.88 inches) with 112.8 mm sidewall.

What affects Tyre Size results

  • Tyre tread wear (a new tyre has ~8mm deeper tread than a worn tyre).
  • Wheel arch clearance when upgrading wheel diameter.
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Frequently Asked Questions

What is an acceptable tyre diameter difference when upsizing?

Automotive engineers recommend keeping tyre diameter within ±2% to 3% of factory original equipment to avoid speedometer distortion and transmission issues.

What does the aspect ratio on a tire sidewall signify?

The aspect ratio (e.g. 55 in 205/55R16) indicates sidewall height as a percentage of tire width (55% of 205 mm = 112.75 mm).

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