Tyre Comparison Calculator
Compare two tyre sizes and see the differences in diameter, width, and speedometer accuracy
Current Tyre
New Tyre
Speedometer Impact
How to Read Tyre Sizes
Tyre sizes are displayed in a standardised format on the sidewall of every tyre. The first number represents the tyre width in millimetres, measured from sidewall to sidewall. The second number is the aspect ratio, which is the height of the sidewall as a percentage of the width.
The letter R indicates radial construction, which is the standard for modern tyres. The final number shows the rim diameter in inches that the tyre is designed to fit. Some tyres also display load index and speed rating after the size code.
Key Measurements
When comparing tyres, several measurements are critical. The overall diameter affects your speedometer reading and ground clearance. A larger diameter means your speedometer will read lower than your actual speed. The width influences grip and handling characteristics, with wider tyres generally providing better traction but potentially increasing fuel consumption.
The sidewall height impacts ride comfort and handling response. Taller sidewalls absorb bumps better but may feel less responsive, whilst lower profile tyres offer sharper handling but transmit more road imperfections to the cabin. The circumference determines how many revolutions your tyre makes per mile, directly affecting speedometer and odometer accuracy.
Why Compare Tyres
Comparing tyre sizes helps when you want to fit different wheels to your vehicle, replace worn tyres with an alternative size, or modify your vehicle for specific purposes. It allows you to see how changes in tyre dimensions will affect your vehicle’s performance, appearance, and instrumentation accuracy.
Most manufacturers recommend staying within 3% of your original tyre diameter to maintain proper vehicle operation. Exceeding this threshold can lead to speedometer errors, potential damage to your transmission, increased wear on suspension components, and possible MOT failure due to inaccurate speedometer readings.
Factors to Consider
Speedometer Accuracy
Your speedometer is calibrated for your original tyre size. When you change tyre dimensions, the speedometer reading will no longer match your actual speed. Larger tyres make the speedometer read slower than you are actually travelling, whilst smaller tyres make it read faster. This can affect your legal compliance with speed limits and the accuracy of your odometer for service intervals.
Vehicle Clearance
Larger tyres may contact your wheel arches, suspension components, or chassis during normal driving or when turning at full lock. Always verify that your new tyres will fit within your wheel wells with adequate clearance. Consider that tyres flex and compress during cornering and when hitting bumps, so static clearance measurements may not tell the whole story.
Handling Characteristics
Changes in tyre width and sidewall height alter your vehicle’s handling behaviour. Wider tyres typically improve grip but may make steering feel heavier. Lower profile tyres provide sharper turn-in response but can make the ride feel harsher. Higher profile tyres offer more comfort but may feel less precise during spirited driving.
Load Capacity
Each tyre is rated to carry a specific maximum load at a given pressure. When changing tyre sizes, verify that the new tyres have an adequate load index for your vehicle. This is particularly important for larger vehicles, vans, or any vehicle regularly carrying heavy loads. Insufficient load capacity can lead to tyre failure.
Frequently Asked Questions
What is the maximum safe difference between tyre sizes?
Most vehicle manufacturers and tyre experts recommend keeping the overall diameter difference within 3% of your original tyre size. Exceeding this can cause speedometer inaccuracy, drivetrain stress, and potential MOT issues. Always consult your vehicle handbook and consider professional advice before making significant changes.
Will changing tyre size affect my fuel economy?
Yes, tyre size changes can impact fuel consumption. Wider tyres typically increase rolling resistance and aerodynamic drag, potentially reducing fuel economy. Larger diameter tyres also weigh more, which can negatively affect acceleration and efficiency. However, the impact is usually modest for small size changes within recommended tolerances.
Can I fit wider tyres without changing my rims?
Each rim width is compatible with a specific range of tyre widths. Whilst some flexibility exists, fitting tyres that are too wide or too narrow for your rim can compromise safety, handling, and tyre longevity. Check your rim specifications and the tyre manufacturer’s fitment guidelines before purchasing tyres of a different width.
Do I need to recalibrate my speedometer after changing tyres?
If you stay within the 3% diameter tolerance, recalibration is typically unnecessary. However, for larger changes, some vehicles allow speedometer recalibration through the vehicle’s computer system. Modern vehicles with electronic speedometers may require a professional to adjust the settings, whilst older vehicles with mechanical speedometers may need physical gear changes.
Will my insurance be affected by changing tyre sizes?
Significant changes to tyre sizes, particularly those that alter your vehicle’s performance or appearance substantially, may be considered a modification that must be declared to your insurance provider. Failure to declare modifications can void your insurance. Contact your insurer before making changes to confirm whether declaration is required.
Can different tyre sizes affect my ABS or traction control?
Modern electronic safety systems like ABS, traction control, and stability control rely on wheel speed sensors that are calibrated for your original tyre size. Significant tyre size changes can confuse these systems, potentially causing warning lights or reduced effectiveness. Staying within manufacturer recommendations minimises this risk.
Should I replace all four tyres with the new size?
For best results and safety, all four tyres should be the same size, brand, and tread pattern. Mismatched tyres can cause handling imbalances and confuse electronic safety systems. On four-wheel drive vehicles, mismatched tyres can damage the drivetrain. Always aim to replace tyres in sets of four, or at minimum, in pairs on the same axle.
How do I know if plus-sizing is right for my vehicle?
Plus-sizing involves fitting larger diameter rims with lower profile tyres whilst maintaining the same overall diameter. This can improve appearance and handling response. However, it typically reduces ride comfort, increases costs, and makes tyres more vulnerable to pothole damage. Consider your priorities and driving conditions before plus-sizing.