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Use our tyre size comparison tool to check how two tyre sizes differ in width, sidewall height, overall diameter and rolling circumference. It helps you understand whether an alternative tyre size is suitable for your vehicle before buying.
Thinking of changing tyre size or fitting bigger wheels? This free tyre size comparison calculator checks a new tyre size against your original one, showing the change in overall diameter, sidewall, rolling circumference and the speedometer error so you know your actual speed before you buy. Enter both sizes below, then find the right tyres for your car or browse tyres by manufacturer.
Enter your current size and the new size exactly as they appear on the sidewall, then press Compare.
Car tyre sizes are written in metric format, such as 205/55 R16. The first number is the section width (the tyre width or tread width) in millimetres; the second is the profile, or aspect ratio, which sets the sidewall height as a percentage of the width; and the last number is the rim diameter in inches, the wheel diameter the tyre fits. Not sure what your current markings mean? Decode any size with our tyre size calculator.
From these tyre dimensions the calculator works out the overall diameter (the full tyre diameter, written as tire diameter in the US) and the rolling circumference for each size, then reports the difference in diameters between your original tyre size and the new tyre size. As long as that difference stays within about 3%, the fitment and gearing stay safe on a typical passenger car.
When you fit bigger alloy wheels, the aim of plus sizing is to increase the wheel size while keeping the overall diameter close to the original. Use this tyre size comparison to test alternative tyre sizes before you buy: a small difference in diameters keeps the rolling circumference, and therefore your speedometer and odometer, accurate.
A larger overall diameter causes speedometer error, where your actual speed is higher than the dial shows; a smaller diameter does the opposite. Our tyre size comparison calculator (also written tire size comparison calculator) compares the two sizes and shows the speedometer error so you know your true speed at a glance.
Once you have settled on a size, compare tyres from leading brands that fit it, then book fitting near you.
Jump straight to some of the UK's most-searched tyre sizes:
As a rule of thumb, keep the overall rolling diameter of the new tyre size within about 3% of the original tyre size. Inside that range the speedometer stays accurate and the tyres clear the bodywork and suspension. A larger difference in diameters can cause rubbing, gearing changes and speedometer error.
Yes. A larger overall diameter travels further per turn, so your speedometer reads slower than your actual speed and your odometer under-counts the miles. A smaller diameter does the reverse. This tyre size comparison calculator shows the speedometer error at common speeds so you can judge the impact.
Plus sizing means fitting larger alloy wheels with a lower profile tyre so the overall diameter stays close to standard. Going up in wheel size by one inch usually means dropping the profile and adjusting the width. Compare the alternative tyre sizes here to keep the rolling diameter within tolerance.
The rim diameter in inches must match your wheel exactly, so only the width and profile can change. The new width also needs to suit the rim width. When in doubt, use the size recommended by your vehicle manufacturer.
You can fit an alternative tyre size as long as all tyres on an axle match, the load index and speed rating meet your vehicle's requirements, and the rolling diameter stays close to standard. Tell your insurer about any non-standard wheel or tyre sizes.
Check pressures, tread depth, speed ratings and more with our quick and easy tyre tools.
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