How much tyre size change is acceptable?

As a rule, the overall diameter should stay within 3% of the original. Beyond this, your speedometer will be significantly inaccurate and it may affect ABS and traction control systems.

What happens if I fit wider tyres?

Wider tyres offer more grip in dry conditions but may increase aquaplaning risk, fuel consumption, and road noise. They also need adequate wheel arch clearance. Check that wider tyres do not rub on suspension or bodywork at full steering lock.

Does changing tyre width affect insurance?

Yes. Fitting non-standard tyre sizes is considered a modification and must be declared to your insurer. Failure to declare could invalidate your policy.

Can I fit wider tyres on my car?

You can usually go 10-20mm wider than standard without issues, but check clearance to arches, suspension, and steering components. Wider tyres may need different offset wheels. Going too wide can cause rubbing on full lock or over bumps, and may affect insurance.

Do wider tyres grip better?

Wider tyres have a larger contact patch which improves dry grip and cornering. However, they are more susceptible to aquaplaning in wet conditions, increase rolling resistance (reducing fuel economy by 1-3%), and are more expensive. For UK conditions with frequent rain, standard widths are often the best compromise.

CarCalcs

Tyre Width Comparison Calculator

Compare two tyre sizes and see the differences in width, diameter, and circumference.

Original Tyre

Width

Total width of the area or object

Profile

Tyre aspect ratio as a percentage of width

Rim

Wheel rim diameter in inches

New Tyre

Width

Total width of the area or object

Profile

Tyre aspect ratio as a percentage of width

Rim

Wheel rim diameter in inches

How We Calculate This

Compares the calculated dimensions of two tyre sizes and shows the percentage difference.

Speedometer Error

The speedometer error percentage equals the diameter difference percentage. If the new tyre is 2% larger, your speedo will read 2% slow (showing 60 when you are actually doing 61.2).

Frequently Asked Questions

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Find out how changing tyre size affects your speedometer reading.

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Convert tyre pressures between PSI, bar, and kPa.

Last updated: February 2026

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