Why does tyre diameter matter?

Tyre diameter directly affects your speedometer accuracy, ground clearance, and gearing. A larger diameter means your speedometer reads lower than actual speed, and a smaller diameter reads higher. The 3% rule means staying within 3% of stock diameter.

How do I measure tyre diameter?

You can measure from ground to top of the tyre, but it is easier to calculate from the size code. The diameter = rim size in mm + (2 x sidewall height). Sidewall height = width x aspect ratio / 100.

What is the most common tyre diameter?

Most family cars have overall tyre diameters between 600mm and 680mm. Larger SUVs and 4x4s can have diameters up to 800mm or more.

What is the 3% rule for tyre diameter?

When changing tyre sizes, the overall diameter should stay within 3% of the original to avoid significant speedometer errors and clearance problems. Beyond 3%, your speedometer reading becomes unreliable and the change may not pass an MOT.

Does a bigger wheel mean a bigger tyre diameter?

Not necessarily. When you go up a wheel size (e.g. from 16" to 17"), you typically reduce the tyre profile to maintain a similar overall diameter. For example, 205/55R16 and 225/45R17 have very similar overall diameters despite the different wheel size.

CarCalcs

Tyre Diameter Calculator

Calculate the overall diameter of any tyre from its size code.

Width

Total width of the area or object

Aspect Ratio

Tyre sidewall height as a percentage of width

Rim Diameter

Wheel rim diameter in inches

How We Calculate This

Calculates overall tyre diameter from the three key measurements in the tyre size code.

Formula

Diameter = (Rim x 25.4) + (2 x Width x Aspect ratio / 100)

Frequently Asked Questions

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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.