Compare two tyre sizes side by side or find alternative sizes within safe rolling circumference tolerance — see exactly how changes affect diameter, sidewall height, and speedometer accuracy.
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If Tyre 2 is fitted but the speedometer is calibrated for Tyre 1
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A tyre size like 205/55R16 contains three key measurements. The first number (205) is the tyre width in millimetres, measured from sidewall to sidewall. The second number (55) is the aspect ratio or profile, which is the sidewall height expressed as a percentage of the width. The letter R means radial construction, and the final number (16) is the rim diameter in inches. Together these values determine the overall diameter, rolling circumference, and revolutions per kilometre of the tyre.
Your speedometer and odometer rely on counting wheel revolutions to calculate speed and distance. When you change tyre size, the rolling circumference changes, meaning each revolution covers a different distance. A larger tyre rolls further per revolution, so your speedometer will read lower than your actual speed, and your odometer will under-report distance.
Most vehicle manufacturers recommend staying within 3% of the original tyre's overall diameter. This keeps speedometer error small and avoids issues with ABS, traction control, and gear ratios. Our tyre calculator filters alternative sizes to within 1% circumference tolerance for the closest possible match.
When fitting larger alloy wheels (upsizing), you need a lower profile tyre to maintain a similar overall diameter. For example, moving from 16" to 17" wheels typically means switching from a 55 profile to a 45 profile. This keeps the rolling circumference close to the original, ensuring your speedometer stays accurate and your car handles as the manufacturer intended. Downsizing works in reverse: smaller rims paired with higher profile tyres can improve ride comfort and are a popular choice for winter wheel and tyre packages.
